I'm usually a very happy person. And then I'll just think about something sad or scary and I usually won't get over it for weeks at a time. It's a really horrible kind of sad and it makes me scared and my stomach starts to hurt. But it's not all the time. When I'm not sad, I'm usually very happy and almost excited. I really don't know if I should talk to someone because I don't know if there's an actual issue.
It is possible you may have bipolar. People with bipolar their moods can change in the blink of an eye. I myself have it. I can be having a great time hanging out with friends and I think one little thought or see a tiny amount of something and it makes me sad and takes all the happiness away. Talk to a psychiatrist, or better yet a friend or family member about it. It can be a nuisance to have bipolar, but just remember that you are never alone.