Well, if you are a teenager which I am assuming that you are, it is normal. Believe me, I've had episodes where I'm happy, and then I literally go into my room and slam the door and scream and cry and plot murders in my head. It's normal. Just take deep breaths and go with the flow :]
Yeah, growing up does that, puberty accelerates it, especially in women. It will probably level off in a year or two, unless you have something like bipolar disorder, but you should check with your doctor on that one.
get into watching yourself. become an observer who is sad, who is mad, who is happy. then ask yourself = who is watching? Hopefully, you'll find the infinite nature of love which is who you actually are. you are not your emotions. you are much more than that. I'm going through the same thing last week: Googled meditation, downloaded some mp3s. put em in my headphones and whallah! stabile again. Never too early or too late to get out of a loop.