I have had experience with this in the past. Like everyone else said, there could be several reasons for this. The first thing that came to my mind was the litterbox. The second was a UTI. My last cat I had this problem with was horrible. We were told by the vet it probably was behavioral and to try and seclude her to stop. Unfortunately, by the time I took her in to have her checked out it was too late. She had infection so bad in her tract that there was nothing they could do. And she ended up with kidney cancer too, and I was told by another vet that the lack of care could have progressed that. I'm not telling you to scare you into thinking your cat could possibly be very sick, I just want people to know that they can't let it go on too long without being checked out. You need to be firm with the vet and get them to test for UTI. If it comes back ok, then you know she is acting out. If not, you will then be able to treat the problem.

Good luck!

Stacy