What's wrong with me?... weird shots of depression...?

As a 15-year-old, you are most likely going through the rapid hormonal fluctuations that teenagers experience. Don't get too dramatic about it. It will likely p over in time. I'm 16 and I also sometimes experience strange, inexplicable periods of depression. Your hormones and emotions are constantly changing. Don't let one little "episode" of depression ruin you. Just hang in there and things will get better eventually. ;)