I got a bad stomachache on Thursday from eating something from a food truck. Felt sick, then felt better the next day.

Ever since Friday I've had this lingering feeling of malaise. Like something's wrong despite (for the most part) everything being ok. This has happened before, I noticed that it happens after my appetite is affected in any way. This feeling is completely maddening. I can handle depression just fine but this feeling is enough to drive me crazy. I'm fine and everything's fine. Why do I feel like something's wrong?