Don\'t Know What\'s Wrong With Me

I really want to find out what is causing me to get really sad so randomly.  About once or twice a month for a week at a time, I feel really upset.  I feel hopeless, helpless, and I just really hate myself.  It feels like there is no escape.   Then, I just start feeling better.  I am not extremely happy, but I feel normal.  I'm not upset anymore.  Then the whole cycle just starts all over again. I feel like I can't control how I feel, like someone is pulling me around and I have no choice where they take me.  If I could find out what the cause is, maybe I could find the solution.  But I have no idea what my problem is. Please leave your ideas if you have any...