I find the doctor frustrating and not any better at finding the problem. If you can...do it yourself. That's my opinion.
Keep a food journal. Write down everything you are eating: meals, snacks, and drinks. And also keep track of how you feel. I'm not sure how long it'll take, but you'll start seeing a pattern after awhile. For some people they find out it's a food allergy, like bananas...or something simple. It's not always simple. Just giving ideas.