maxx's mom, I would try a different vet if she resumes the behavior. Cats are extremely fastidious, and even a cat with behavior problems will at least attempt to squat and cover, because that is how they protected themselves when they were wild. She would not be doing this, even if she were mad, if she were well. In fact, to me it sounds like she may have a tummy bug such as coccidia or giardia.

I hope she's better, but if not, I hope a new vet can help her.