Why am I so on and off?

One moment I will be smiling and laughing and having fun. The next I will be insanely depressed and I really get to points of hurting myself, not like cutting but like recently I held a drill to my temple at full bore and did some damage and it will scar. I don't know why I did it, it was on impulse. I don't think I'm bipolar by any means, I just don't really know what's going on.