Well does she have any other symptoms with it? Cry when using the litter box, or going outside of the litterbox? Lethargic (I know, hard to tell with cats) or act in pain in any way? Diarrhea?
There is a possibility she could have infected anal glands. You need to get her to a vet asap for treatment. If you have a feline specialist in your area, that is the best bet.