I have terrible mood swings... what could it be?

I have frequent mood swings, but it isn't like being bipolar. I mean... I am either really happy, pissed off or sad. It is like... I will get upset at the smallest thing. My boyfriend says I can help it, but it is too much of a compulsion to catch myself before I respond. It is not like I am an angry person though, 'cause I am generally a nice person, but on any given occion I can just freak out. It isn't even on a special occion either... usually it is unprovoked .. like someone will ask me a question and all answer all pissed off, but after a while I will be fine and then later in the day it will happen again. I don't know... the mood swings are too frequent not to notice.