Why am I feeling so down? What's wrong with me?

I've been having down spouts for a while now, I get really upset, but I don't know why. Sometimes it can be little things like just being moody, but sometimes it goes as far as me wanting to harm myself. I've harmed myself before and the want to do it again is increasing. Some times I just cry, but I don't know why. These moods don't last long and the majority of the time I'm happy, but they're increasing in length and they're getting worse. Am I just being hormonal? Is this just a phase?