This is something that has been in my head for weeks. So I wrote the hottest parts down. Contains body modification by... somebody, reversed digestion, pleasant scat, terminological confusion that would give Escher a boner and a heartfelt desire to bring our troops home. 
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Nela's eyes opened suddenly, then just as suddenly snapped shut again accompanied by a gasp of pain. There were bright lights, and her eyes felt like they hadn't seen light in months. Her body ached, like she'd been running marathons for just as long. Her memory was foggy. What was going on?
She felt around herself, finding that she was laying on a bed with no sheets. She couldn't remember what her bed felt like, but somehow this didn't feel like home. Slowly she tried to open her eyes again, found the light was rapidly becoming less painful, and gradually opened them wider a millimeter at a time until she could make out her surroundings. She was staring straight up at... bare fluourescent tubes. Like in a warehouse, not a home. Nela turned her head to the side and, seeing the light was only above her, opened her eyes the rest of the way. After a moment for the blurs to melt into meaningful images she was able to make out a flat, white wall, illuminated only by the light hanging above her. She traced the wall around the room with her eyes, stopping with a gasp, followed by a horrified wail as her gaze swept across her own body.
Of course in her memories she could see the image of herself from thousands of occasions before. Her thin, reptilian body, covered in green scales from breastless chest to tailtip was a familiar daily sight, but not with the enormous surgical scar stretching from her abdomen up to her chest, and presumably up to her neck judging from the sensations she felt. Someone had performed surgery on her - and this didn't look like any hospital recovery room she'd seen before. Panicked, Nela rolled off of the stiff mattress, landing on her feet and then stumbling, collapsing on weakened, atrophied limbs. She grunted as she landed, catching her own fall with her hands, then noticed for the first time that her handclaws were missing. Frantically she tried to drag herself across the smooth linoleum floor, the scales of her clawless fingertips clicking as she scrabbled like a cartoon character, unable to get any purchase. Terror rose in her, followed and then obliterated by adrenaline. She had to get out, get away, escape wherever this place was. Hissing, she poured every ounce of energy she had into forcing her weak legs to push her up and into a standing position, and as she struggled to her feet she swept her eyes around the room, her sharp reptilian eyesight picking out and analyzing every detail, of which there were few. The only obvious opening to another area was a door in the wall furthest from her.
Nela lurched towards it, stumbling at first but quickly regaining her balance, crawling towards the door half a foot at a time until she got close enough to reach out and touch it. With only a foot to go she fell forward, landing against the door and sliding to the floor with a clunk. She reached up and gripped the knob, turning it and pulling... only to find it locked from the outside. The adrenaline rush eliminated by defeat, her escapee rush turned to despair, and tears rushed to her eyes. Nela started to cry, the fear, anger and frustration too much to bear now that she was sure she was trapped here until whatever psychotic cult that had kidnapped her decided what to do next. Maybe she'd been taken by organ thieves, and that bare mattress was meant to be her deathbed. Or pharmaceutical company agents performing illegal experiments on sentients. Or--
Her thoughts were interrupted by a sudden wave of... something. Nausea, almost, but although it felt exactly like she had to throw up there was no pain or discomfort associated with it. She just felt like she needed to empty her stomach. Nela let herself fall forward onto her hands, closed her eyes, opened her muzzle and waited for the heaving to start. After a moment she felt her abdominal muscles start to tense and release, like the early stages of a dry heave, but as they continued she suddenly felt something solid in her throat, moving upwards. She was puking... slowly? Nela opened her mouth wider and tried to flex her diaphragm to push the mass out faster, then suddenly a wave of the most rank fecal stench drifted through her nasal passages. She thought she would gag, but then realized with a shock that the smell didn't make her feel sick or disgusted. It was just a smell... and maybe she even liked it a little. She didn't have any more time to think about that inexplicable event as the lump in her throat suddenly slid into her mouth and dropped onto the floor. She opened her eyes tentatively, hoping it wouldn't be a gigantic mass of bloody tissue, only to see... a hunk of shit.
She'd just had a bowel movement in reverse. Before she could think any further on this bizarre event, her ears were flooded with a voice projected from an unseen speaker.
"Subject 121 is awake and alert. Release immediately."
Instantly there was a loud click from the door, and Nela wasted no time grabbing the knob and wrenching it open. She pulled herself out the door, into the blinding light of a sunny city street. As the knobless, unmarked door closed behind her, the lock clicked automatically.

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Nela smiled at her boyfriend, Gale, a brown lizard of her own species, as he kneeled at the foot of their bed. He lifted her legs into the gynecologists stirrups hanging from the ceiling, spreading them wide and exposing her vent. Normal reptiles of her species would have had only a tiny, thin slit in the scales of their crotch, but years of this kind of routine, at least three times a day, had left her cloacal opening loose and gaping. Gale picked up a bowl of hard-boiled, shelled eggs from the floor and set it next to her on the bed, then picked one up, smiling back at Nela as he placed it against her vent, then pushed it inside. She sighed as the cold ovoid popped inside of her. Feeding had not reduced the sensitivity of her reproductive orifice at all, which made this ritual so much more enjoyable than it could have been. Another egg was pressed against and into her cloaca, and then another, until the fourth stopped partway in, the narrow end protruding from her vent. Gale rose to his feet, revealing a hard and dripping shaft hanging from his own vent. He stepped forward, pressing the tip of his shaft against the exposed end of the egg, then gently thrust forward, applying gentle pressure to the egg until it popped into her, eliciting a pleasured yelp as the first egg he'd pushed into her slipped through her sphincter and into her esophagus. He continued to slide his cock into Nela's vent, pushing each egg in turn into her throat until he felt his own cocktip gripped by her internal anus. To keep the eggs from sliding back out he stayed inside of her while peristalsis pulled them into her stomach, groaning as her tight throat muscles massaged the tip of his shaft.
Once the eggs were well into her digestive tract Gale leaned forward and began to thrust gently into Nela's loose vent, the stretched walls no longer providing the satisfying sensation they were designed for but still providing enough to keep him hard and feel enjoyable. As he mated her softly he leaned down and licked at Nela's snout affectionately, and Nela licked back, knowing what he wanted and opening her maw. Inside her muzzle dirty feces were caked over every surface, her teeth were stained and filthy, and her tongue was brown from end to end. At the back of her maw was the cherry-red flesh of her rectum. She blew into his face, and Gale moaned in a coprophiles ecstacy as the smell of old, fermented feces blew directly into his nostrils, making his cock leap and twitch inside her loose vent. Though they normally would enjoy this longer, today Gale had been thinking about this moment since he woke up and he could no longer restrain himself. He reached up with one hand and plunged two fingers into her rectum, twisting and pumping them until the red flesh started to convulse. He pulled his fingers out of Nela's intestine, wrapped them around her neck and then, opening his muzzle, pressed his maw tightly against hers, waiting for what he knew was coming. Within seconds he felt her neck start to bulge as a solid lump traveled up through her colon, and he licked at her rectal flesh until he felt the tip of the mass sliding into her mouth. The pair kissed deeply, tongues intertwining and licking at Nela's waste, Gale swallowing as it dissolved and liquified until the entire mass had dissappeared into his throat. As he swallowed the last tiny lump Gale exhaled through his nose, and the wave of his lovers filth pushed him over the edge, his cock twitching over and over as he pumped his watery seed directly into Nela's throat.