It DEFINITELY sounds like a case of hormones to me, particularly because (a) she's never done it before, and combined with (b) the fact that now that she's doing it, she's nearing the six month mark in age...when most buns' hormones start raging.
So, don't worry...if you get her spayed soon, she might just stop doing it with the loss of the hormones she's now experiencing.
Also, if you're having a problem with her moving around and/or dumping her litterbox, you can always buy some binder clips and clip it to her cage.
Another thing we do, is get a clasp and clip the binder clip to the box, and put the clasp on one of the binder clip's arms. Does that make sense? We only do that if the litterbox doesn't meet the height just right of one of the bars, though.
Hope that helps!
OH! One other thing you can try...putting hay on top of her litter, so the litter isn't as accessable.
Hugs!
Rosie*