Relapse?

Do you ever feel like your depression is coming back? I self-harmed last year for a time, but got help and have been clean. I don't want to sh but I feel like I'm going to fall back into depression even though I'm happy. I truly am happy-I am working at our school to create a peer counseling and suicide prevention. I'm in chorus and the girl I had drama with last year left. I just feel a little sad. Why?