You stop short after leaving the stall, thinking on the matter. If you wanted more people up your butt, then the best way would be to surprise someone while their guard is down. And, most people's guard would be down right as they enter a bathroom stall.
With that you pivot and go right back into the stall you were in, shutting the door. You decide to step up onto the toilet, giving you not only a decent view of the locker room by peering over the stall walls, but when it came time to surprise, all you'd need to do is turn around and lean back onto an unsuspecting victim's head. It was seemingly perfect, now all you had to do was wait.
As it turned out, you didn't need to wait too long, as a woman entered from the pool side, a worried look on her face. Odds were, she'd pick your stall, so you turned around, butt facing the door, and waited. Sure enough, the door swung open, and the women absentmindedly stepped forward, but was taken from her harried mind by her face bumping into your plush ass. She barely had time to say a "what?" Before she was plunged shoulder deep inside your butt. Her struggles prove fruitless, as you clenched powerfully, enough to lift her feet from the ground and curb her struggling. A few more clenches left only her feet remaining, but of course those, too, disappeared.
By now, shoving all these people between your cheeks had started to show on your physique. Evidentially your stomach had started to break down your victims, including your dear mother and Charlotte, and your butt had expanded to a monstrous size, compared to your small body. It really turned you on, the thought of digesting so many people. You needed more.
Afterwards, you stayed in the stall, shoving any who dared to enter up your ass. One, then two, one time two at once, making four. Time went on, spreading your cheeks and feeling your ass expand again and again. After nearly an hour, you sat down and breathed heavily. You were up to 8... No, you'd shoved nine people up your ass now. Your stomach was big, sure, but by now most of your victims were digested, and your ass had grown to mammoth proportions. It had to be at least half your body weight now, your cheeks were like two bean bags in size, and trying to stand up felt a bit like exercise. Nevertheless, you got up and began walking out, finding it easier once you started walking. Every step you took made your cheeks bounce up and down, and you could've sworn you could hear your ass sloshing. It was a great feeling, but you felt like you needed one more thing... That was when you saw the shivering boy again. You couldn't believe he hadn't left the locker room yet, until you saw his boner. He was staring at you belly and ass in awe. You smirked, and decided he'd be of good use.
You asked him how he liked your body, turning around and squishing your butt cheeks. He didn't respond, just stared wide-eyed at your features. You suggested he get a closer look, but as he leaned in you grabbed in and shoved him between your cheeks. You made sure not to shove him into your butthole, just to trap into in your crack. You fit him in quite easily, and before long he disappeared, like he'd never been there in the first place. With that, you sauntered away, back home, taking your dump truck ass, and your passenger, away.