When people have stomach ulcers, irritable bowel syndrome etc. you need to do a 'trial and error' approach. You basically need to try different foods and see which upset our stomach and which do not. It's different for every person.
Generally speaking, most people with digestive issues will want to stay away from wheat, soy and dairy as those foods are very inflammatory in the body.
Most people with digestive issues thrive on a diet based around fruits, vegetables, organic meats, free range eggs and wild caught fish.