How old are you?
Do you have a history of mental illness in your family? How long have these mood changes been going on?

In my opinion, you probably shouldn't automatically attribute your mood swings to bipolar disorder. Like women, men go through progesterone cycles every month, and when progesterone levels are at their highest, mood swings, agressiveness, and depression are common occurances.

If the mood changes continue to occur for a month or more, you should consider seeing a psychiatrist.