Based on plenty of experience with UTI's I can say that they will usually pee outside the litter box and not poop. Having said that it is still worthwhile to take Priss into the vet just to be on the safe side.

The only other thing I can think of is when you cleaned the litter boxes (to switch litter) what type of cleaner did you use? I know that mine don't like anything with a "strong" smell (even though they can easily out smell it ). They have me trained to use a plain anti-bacterial dish soap and I don't seem to have problems with that.

Good luck