well, without knowing more about you, i can say that sometimes a particular incident can set a person feeling like this (even as a delayed reaction) ... other times there doesn't seem to be an obvious reason for it. maybe you could try doing some different things over the next few days you've always wanted to do, or something you love but haven't done in ages, and see if that lifts you a bit. if it doesn't help i could only suggest you go see a doc ... and keep posting here for support if you need.