I've done the fist-clenching, nail-digging (and hand-slapping) a couple of times. But that was when my mind was basically running away with itself, making me think of stuff I didn't want to think of. At the end of the night I told a friend what was bothering me and that was a huge relief and I can now think straight again.
So, I guess my point is: ask her about it. I'm pretty sure if she's doing that something's bothering her, so try to get it out of her, provide a listening ear, perhaps some advice. And who knows, that might do the trick.