If you're feeling sick to your stomach yet doctors can't find anything wrong with you, it may be something you eat. Here's a story by Ed Rockey for the Los Angeles Times, where his food allergy was even misdiagnosed by a doctor as being depressed:
"We've done every test I can think of, and there's nothing organically wrong with you," said the internist. "I think you are clinically depressed. I'm putting you on an antidepressant." I had an intuitive sense I'd been misdiagnosed. Had I ever felt depressed? Sure. But the symptoms I had reported to him didn't feel like depression. I asked him about the side effects of the drug he prescribed. I refused to take it. As I left, he warned me, "You'll be sorry."
A couple of months later, I was in the office of a dermatologist for a routine checkup, and I had one of those attacks of nausea and weakness. It was the first time I'd had those dreaded symptoms while with a physician. He checked my vitals. Blood pressure was way down, for one thing. He got a hunch. "I once had a patient with symptoms just like yours," he said. "Turned out he had food allergies. Why not get checked out for food allergies?"
An endocrinologist ran tests, which indicated I was allergic to several foods. Results came in a flow-chart format, displaying levels of severity associated with my allergies. The most severe: cow's milk. The next most severe: wheat.