Maybe she has an ulcer? I've been reading about them after my equine chiro said my guy likely has one. They can have weird symptoms like attitude changes and crabby / nastiness for no obvious reason.
I don't know but that must be frustrating and even a little scary! To have a horse you trust so completely do those things must shake your confidence when you deal with her. Unpredictable is rough!
I hope you find out what is going on and find a solution for it quickly! It sounds like something is certainly off with her and she'll probably feel much better once you can find and fix it!