is it all foods? or just certain foods? It could be a lot of things; acid reflux, a food allergy, some sort of
IBS, gastritis, an ulcer.
The foods that usually cause such things are foods highly acidic (pasta sauce, coffee, tea), dairy products, garlic, spicy foods, gluten. All these foods have a habit of irritating the stomach. try to avoid them and see if that helps. If not, go to the pharmacy and pick up some nexium and see if that helps.