I've had constant nausea & stomach pain for years with not much relief, and nothing has shown up on ultrasounds or xrays. what could be causing this?

Too Complex to say: Abdominal pain and nausea can be caused by many different diseases. if you had x-rays and ultrasound, you have seen a doctor or an assistant (nurse practitioner or physican's assistant). In either case, a complete history and physical should have been done. Without that information, I cannot make even the start of a diagnosis. Ask your doctor.