Why didn't Nikki shit and piss herself more in the movie

We're all familiar with the iconic scene in which Nikki pisses and shits herself in the living room but I've been thinking ISN'T IT a bit logically inconsistent that she only freezes in place and pisses and shits all over the floor and pukes on her chest all nasty and stinky that one time, and not every single time she's away from Bear?

Like shouldn't she just be covered in vomit and urine and feces every time he returns to her throughout the film after he makes the wish? Am I supposed to believe she was just going to the bathroom on her own and not soiling herself and barfing on her own chest every time but then randomly didn't do that the one time for some reason?