Why does eating make me so tired?

I snack more than I eat regular meals, but I do eat at least one meal a day. For the past year or so, every time I eat a real meal I’m immediately exhausted. It doesn’t seem to matter what kind of meal it is… a dinner salad or a full breakfast both have the same effect.

I don’t have the best diet in the world, granted, but my snacks aren’t terrible, either. What’s up with this?