Say that then. "I don't know what is causing this. I've ruled out everything I can, you may have CFS or psychosomatic symptoms, but I can't test for either. Here are some resources on both. Psychiatry has helped x% of patients with your symptoms, I can't guarantee it will help but here's a referral."

Much better to say "I don't know" than to give a confident diagnosis just because you've run out of things to test for.