If I'm not mistaken about the feeling you mean, I get this problem when I'm depressed (I mean bio-depressed, i.e. messed up emotions due to some physiological factor) while not having very much to be sad about.

I have dysthymia and antidepressants have helped. However, I recently had to change one of my other meds, and I've been getting the problem again due to a med side effect.