Am I developing depression?

Sometimes I just feel so down I want to cry, and I don't even have a particular reason. I have mood swings, from being fine to extremely cranky and irritable. I'll think I need a particular thing to make me feel better, that that one thing will make me all better. Often once I get that thing I feel no better, and if I do feel better its only temporary. And I often get very insecure in some social situations, and can be extremely reserved and shy. Are these the sign of depression?