The list is long. See your physician for a work up rather than guessing and trial and error.
Although I hate to say it, stomach pain and upset can be caused by a number of different things, conditions that range from mild to those more serious. Have you consulted with your doc, or tried to figure out if foods or medications might be contributing to the problem? What have you tried so far? Any underlying medical conditions? Do any GI issues run in your family?