Well it could depend what you're eating? What do you eat. Also, if you wait until you're REALLY HUNGRY BEFORE eating that could cause stomach aches too. You want to be hungry when you eat, but not super hunger.
Could be many things, i.e. lack of or too much hydrochloric acid, not enough digestive enzymes, lactose intolerance, gluten intolerance, H pylori or other infection, gallbladder problems, parasites, over consumption, GERD, tight clothing, etc etc.