So you're holding the hook grip even overhead? Was that a conscious decision, or is it just happening? If you haven't tried it already, try releasing the hook. If you're sticking to hook because you snatch better with it, never mind.
Try 2 things:
1. stretch - make the girl punch fist (i.e. make a fist around your thumb like a hook grip with no bar) and stretch the hand away from the thumb. Also general wrist mobility work and forearm stretches.
2. Ice the shit out of it.