Could I have a mental illness?!

For a good few years now I have been having trouble in my life. It is quite difficult to explain but I'll try my best. I feel that there's long periods in my life when I'm severely depressed and thinking suicidal and angry thoughts. Most of the time without any reason whatsoever. It's very difficult to cope with and it has sometimes been effecting my relationship with my friends and family. During these periods I also lose all interest in things that for the other periods of my life (explain more later) I thoroughly enjoy and look forward to. But then there's other times when I feel elated and fullfilled. Sometimes to the extent that I'm doing the most stupid and ridiculous things. I think it's a problem because of how severe the mood swings are, and for such long periods of time. I feel that this is impacting my relationships with friends and family in a bad way and even though I am going to see the doctor I was wondering in the meantime anyone had any ideas what could be the problem.