What's wrong with me?

For some reason, nothing seems fun anymore, even activities that used to be my favorite. They just seem dull and uninteresting. Same thing with my friends. I never want to go to movies or restaurants with them, i just feel like none of them like me and that it's not fun when I'm with them. I'm always really tired, and lately I have either been extremely hungry or not hungry at all. What could be the matter here?