Could I be bipolar or something?!

Over the past few years, I've had a quite radical personality change. I don't know what it is, and it definitely does not happen normally to my other mates. Anyway, I've been having feelings of being useless, having no point to life and just feeling generally so down that I've considered leaving everything and just not bothering any more. This goes on for about two weeks at a time, after which I get the opposite. As sure as I woke up this morning, I suddenly feel really great about everything, so optimistic and happy, like nothing is wrong in the world. But sometimes when I have this "happy" period, I do things that could end up potentially bad, like spending money on things I don't really need, or having a go at people and nearly starting fights for no good reason. On one occasion, I scared a couple of friends with really dark things that I said. As I go between these two moods, I go from loving my friends to just looking at some of them and thinking really negative things about them. I don't like what happens when I go through these stages, but it just happens. I can't control it, its like a whole different personality comes out. I've never publicly let my feelings out in a bad way, but I don't want to wait until I do. Any advice on what could be wrong with me, how to overcome it or anything? Thanks :)