The feeling in your gut worsens. It's not quite painful, but it's definitely unpleasant, and you double over for a moment. As you do, you get a glance at the box you took the costume out of, and you see that you hadn't looked closely enough at the label. What you thought was a "bear" costume was actually a "teddy bear" costume. You have enough time to wonder what difference it makes, costume wise, before the feeling overtakes you and you fall back onto your butt. As quickly as the unpleasant feeling started, it stops, and you try to get yourself back up. Your legs won't respond, though, and when you look down to see why you realize your uncle wasn't kidding about the magic.
Your legs are splayed out in front of you, but you wouldn't exactly call them your legs anymore. They've fattened out into fat cylinders, and when you reach down to touch them, you find that they're filled with stuffing. You can press down harder and your hand sinks all the way into them, you can't feel any bones or structure at all inside. You look down to where your feet should be, but you don't have any now, your legs simply end in a flat surface. You try to get your legs to move at all, but although you can still feel them, they refuse to respond to your commands, and stay splayed out directly in front of you. You look in the mirror and see the changes are still going, sweeping up your body, making you lose definition and barreling you out into a cute, chubby shape. You try to swing your arms up to rock yourself upright but when you put them in front of you, they lock in place, and soon they're fat cylinders just like your legs. With nothing left unchanged but your head, you're forced to watch in the mirror as your mouth stitches over into a tiny cute smile and your head puffs out to be the width of your shoulders. Your eyes glass over, although you're thankful you can still see out of them somehow, and your human ears vanish, being overtaken by the bear ears on the headband. The fur creeps up over the last of your hair before finally stopping. Your changes complete, you try to shout out, to move, anything, but you just sit there on the ground in a pile of costumes.
"Hey, kid, you coming back?" After a few minutes of internal panic, your uncle sticks his head in the door. "Now where did he get to...man, this place is a mess. I really have to reorganize it." You desperately try to signal to your uncle that you're sitting right there in front of him, but he doesn't notice you. He gives up on his search really quickly, not even noticing how unusual it is that a large teddy bear is sitting against one of his shelves. Turns out your uncle is a lazy, inattentive man. "Damn...I guess I scared the kid off by talking about magic. Wouldn't be the first time, I guess. Ahhh well, not like I really wanted to take care of some kid I don't even know for a month, anyway. He got here by taxi, I'm sure he can find his own way home. I've gotta get going if I want to catch that three-day wizard's cruise." You're furious that your uncle was planning on ditching you, but the anger fades into a sense of powerlessness over the next half hour as you watch him pack up and head out the door, leaving you in silence. Eventually, you get "sleepy", or something like it, and you find yourself zoning out, hoping against hope that in the morning you'll somehow find a way out of this.