Why don't I feel good after I eat?

I seems as though after I eat, I feel tired. When I am hungry I fell fine, and don't feel tired at all. This doesn't make much sense to me, and my head hasn't felt right as well. I feel like I can't remember much and concentrate that well. My heart rate has noticeably been higher and my breathing is a little shorter. I don't have a fever. I just want to know what is going on.