Depression

I have been severely depressed lately and I have no idea why..me and my boyfriend are happy and we talk a lot about the future..I have all As in school including in my nursing class..I either feel really angry and irritable or really sad and depressed..I eat a fairly regular diet but I wouldn't say healthy I do cheer after school 3 days a week..I have friends but I've recently lost one due to an argument..I take birth control everyday (mononessa)..ive gained weight and my body is always hurting..I don't understand why I get so upset like I started crying today from looking at a newborn baby my teacher brought in and when I was talking to my mom about lunch I cried..help?