We had a male cat few years old, he is the one in my avatar, had to take in the vet this past summer because he stopped using litter box, vet said he did not have anything wrong, no infection or anything, the best that we could figure out is something spooked him where the liter box is, we have another cat and she uses that liter box fine but our male cat just quit, had to get another liter box just for him to use :( Cats are just weird sometimes.
So my question would be, have you moved the liter box from where it was? Is there anything where the current liter box is that could scare her? I would move the liter box to the room where she is going to the bathroom at, till i could figure it all out...it kinda sounds like a emotional problem. But it would not hurt to take it to the vet just to rule out anything physical, an infection would certainly make her do that.