First, Welcome to Pet Talk!!
Bathroom behavior can be such a difficult problem to diagnose because there are so many reasons that can cause a kitty to eliminate outside of the box.
Does she seem healthy aside from the inappropriate elimination? Does she seem to strain at all?

BigCharles mentioned change and he's absolutely right...try everything you can think of. If anything..even the slightest little thing changes in a cat's environment, some cats can become upset...and they'll let you know by going outside of the box.

Any changes in your family?
Any changes in food/litter?
Is the litter clean?
Is the litter box in a quiet and isolated place?

So many questions, huh?!!

Do let us know how she's doing and good luck!
**hugs**