What is this illness called?

I have no idea what this is, and I'm to scared to go to the doctors or tell anyone. How I can explain it, is that I can very easily change what mood i'm in. Say I could be lying in bed and just like that i can get depressed, but then while at school I'll be quite happy, almost hyperactive. But then my mood can change from the smallest of things. I have 2 family members who have bipolar, so I'm not sure whether that could of been passed down to me as well.