There has to be something going on! Cats by nature prefer to use a box, thats why they are trained by instinct at such a young age. You have to find what's winding her up. It may be as simple as moving the box or a different box. Perhaps the litter isn't comfortable to her. My boy will get mad and p*ss all over everything, but he will do the same if given half a chance for no reason at all. Just like last night my son left his towel in the floor after getting a shower, sure enough I picked it up and it was drenched with pee
believe me when I say I know how you feel! My main question would be is she declawed, along with the questions Brian has asked.