Terrible Anger When this happens..?

I do not have an anger problem as I'm usually a pretty chill person, but a small group of things set me off. The first and most severe of these things is anyone hitting my head or face. the other night i put my foot through an old computer monitor and could hardly walk or talk for 10 minutes due to overwhelming rage because of the fact that i hit my head off a shelf. my ex punched me in the face and i screamed in hers and started laughing. I don't know why this happens but When i hit my head it sets me off and i just want to hurt something or myself. can anyone tell me why this is? I dont get pissed at ANYTHING else really and I do not have an easy temper.