incredibly odd mood swings, constantly?

i have some really odd mood swings, and my friends have started noticing and commenting, which is kind of worrying. I am a girl 17. I can often be quite aggressive/angry, and would say in general im a pretty moody/angry person. I was eating out the other night, one second i was really happy/exited and wanted to meet up with more friends, etc. The next second i was feeling tired, bored, unhappy etc!! My friend kept saying it was odd how my mood changes so quickly and stuff. This has happened a few times, and i will go from being really sad and bored etc, to incredibly happy for no reason.... why is this?