I'm not sure what to suggest either beyond getting a referral to a paed to investigate things like diet, chemical imbalances, etc. Might also be worth investigating with the school if they have student counsellors who might be worth a chat with. How about her teacher? Have you discussed it with him/her?

Mainly though I just want to say you're a great mum. We all lose it at times and while none of us are proud of it, the important thing is that you know its not going to be a productive way of dealing with Miss and I'd put the house on it not happening again any time soon. I'd also bet that you're feeling a whole lot worse about the situation than little Miss is.

Sending you heaps and heaps and heaps of hugs and love.