Hmm....I'm not sure. It does sound a bit like Bipolar. How depressed do you get? How long does it last, and how long does the happiness side last?

This isn't professional, but try this quiz:
http://psychcentral.com/quizzes/bipolarquiz.htm

Or, try the Sanity Score, it's pretty accurate:
http://www.sanityscore.com/

Print out the quizzes and take them with you if you go see a psychiatrist. Wouldn't hurt to go, just to get evaluated and see what their professional opinion is.