Mark fell into the toilet with a big splash.

"Okay, that was weird. I don't remember putting the exit portal above the toilet, gross."

His clothes seemed to also have dematerialized after passing through the portal, which was another bug he would need to fix later. After all that build-up it seemed like the experiment was a complete failure. With a heavy sigh Mark decided to portal back home. He had no desire to stay naked and shrunken inside a toilet. He would portal to his bedroom, shower, and put on a clean change of clothes. It was Saturday so he could always try the mall again tomorrow.

The device was still in his hand underwater. He pulled it out and was about the set new coordinates but the display was scrolling an error message.

"OVEREXPOSURE TO H2O. SPACE-TIME FLUCTUATION DETECTED. TIME IS REVERS-"

And then it shut off. This was bad. The device could luckily self-charge over time but that could take hours. Also what was with the that error message? Mark was thinking about it but then the toilet water began to stir. The pool around him was draining and he was being lowered towards the bottom. Oddly enough the streams of water coming from the upper rim weren't filling the toilet, but rather sucking up the water. But that didn't make sense. Water can't just flow upwards by itself in open air, that's impossible.

Then it clicked. Time was moving backwards around him. That's what Mark's device was trying to tell him. It had malfunctioned and now everything around him was happening in reverse order. A giant figure of a girl walked backwards into the stall closing the door in front of her. Her movements seemed odd to Mark as they were all backwards. She bent down to place her purse on the floor when really she was just picking it up before leaving. Her giant butt stuck out above the toilet bowl and Mark recognized the words "NERF THIS" emblazoned across the yoga pants. It was the blonde girl from earlier! But if Mark had just seen her leave the bathroom and time was moving backwards then...

"Oh shit!" Mark yelled and a twisting tornado of murky water and dirty paper emerged from the toilet drain. His half an inch tall body was swept up in the disgusting mess of piss and shit. Huge masses of crap collided with him, threatening to hold him down under the yellow water. The violent twister ended as quickly as it began and Mark used the calm moment to desperately climb a floating wet mound of toilet paper.

Mark coughed up some pee from his mouth, he had nearly drowned. Breathing in for the first time made him gag. The smell of such a enormous amount of poop was overpowering. Compared to him the logs were as big as tree trunks. Looking skywards he saw the girl reaching to flush the toilet, which was the cause of that horrific event.

Then she turned around and peeled off her tight pants exposing her massive ass. Really it was her putting her pants back on but it looked nearly the same. Mark normally could appreciate how sexy her perfect ass was, how shapely it looked while being so huge, but right now he wanted to cry. He knew what was going to happen next. This was not what he wanted.

The giant ass lowered and covered the toilet seat, along with covering Mark in darkness. The only way he could see was a small beam of light coming from between her soft legs and pussy. Her massive cheeks spread open as she sat and he could see her puckered butthole. It winked at him as if to say "hello, I'm going to be eating all that yummy crap you're standing in." Mark knew it was only a matter of time.

He tried yelling "HEYYYY! DOWN HERE! HELP!" but there was no way she could hear his small voice.

More light filled the bowl as a humongous hand came down. The toilet paper Mark was standing on knocked him off. It levitated upwards and returned to her grasp. She rubbed it against her anus, and when it was done the hole had become more filthy. She returned the paper to the roll as if nothing had happened and repeated the process, smearing her backdoor with more and more shit and giving her pussy a quick wipe. Soon Mark was left treading the pissy water, he had not dared to climb any of the bundles of crap that floated nearby.

The asshole looming above twitched and then gaped itself open revealing the girl's dark colon within. Mark gulped at the sight in fear, he knew what was about to happen.

A giant turd next to him rose out of the water with a splash. It made contact with her anus and snaked it's way back inside. Soon more monolithic masses of feces flew out of the lake of pee and up to the ass above. Mark saw up close how a girl's asshole flexed and contracted when she was squeezing turds out. The wrinkled hole was eating shits at a quick pace, chewing the logs as they were sucked within. Mark had to dodge the shit or risk being stuck to one and pulled inside with them!

----

Back in the normal flow of time, the girl was sitting on the toilet patiently letting her ass take a big dump. Her name was Jen, and she was totally oblivious that she was torturing the poor boy lost under her butt, essentially drowning him in her shit. Her plump ass cheeks were so big that they couldn't entirely fit on the seat, they spilled over the sides. Wiggling her hips a bit she spread her ass more on the toilet. She let out a soft sigh, relieving all that built up pressure inside felt good. It felt like her digestion cycle had been acting up and now she could just let loose.

----

Returning to Mark's experience, to him it looked like most of the excrement was gone. The burning smell was going away as the poops returned to their home. Mid-dump farts were sucked inside the bulging anus creating a loud inverse thunderclap. Even the filth around the rim of her anal ring was slurped inside and soon the dump had never happened.

Mark was still at the mercy of the giant girl, stuck wading in her gross lukewarm yellow water. The pool rippled and began to splash into a powerful stream of pee that shot upwards to her pussy. Her small urethra drank up the piss with vigor. The pillar of liquid pulled in the yellow colour from the bowl and sent it shooting back up inside her. Mark panicked as he realized he was being pulled along with it. Swimming for his life he flailed and kicked away from the torrential tower. He was seconds from being swept away in it before the stream stopped.

Given the brief moment of respite Mark tried to catch his breath. He was tired from all the swimming and was glad the ordeal was over. But what would happen next? The tiny high-schooler thought about ways he could have ended up in this toilet. Looking at the giant ass above made him think of some scary theories. There's no way he could have fallen from her butt above, right?

Wrong. Gravity acted in reverse and he was thrown up into the air, just like the disgusting mess the girl had made. Mark screamed as he was flung towards the ass, the fleshy ceiling of girl butt was closing in on him. But where did he go? Or rather, where did he come from? Was it the stuffy pussy or the dark ass crack?