Why do I have these crazy mood swings? Note: I am not bipolar

As far as I know, I am not depressed. Both of my parents work in the medical field and know about mental illness. It's just that I have these crazy mood swings from time to time. I'll feel crazy depressed (Never to the point of suicide) but I tell myself I don't have any friends, I'm fat, im stupid, etc. this lasts for a couple days, and then I go to my usual happy self and spend time my many friends and family. This has been going on for awhile and I don't know what to do about it!! Someone help?