Why is it that after every meal be it large, medium or small i feel a slight stomach pain that lingers all day?

the pain is also present even when i don't eat, in between meals.I also become a bit tired and i feel heavy but diabetes tests have come back negative after multiple tests. It doesn't matter what i eat or drink even from low glycemic index foods to cola or coke i must feel this discomfort after consuming any foods or fruits. The magnitude of the discomfort varies with the type of foods ingested but not the quantity. I have done scans and don't have appendicitis or ulcer. someone please tell me WHAT COULD BE WRONG?