Oh, God, Anna. I know what you mean about independent 'children'- I raised my daughter the same way, and boy, did it take!BUT I agree with the advice above. Go with her, take notes, tell her you would do the same for and with anyone you love (that always works for me; my daughter is fond of telling me that she is a grown woman... I suspect your independent daughter is also a logical thinker- it seems to go with the territory- so taking notes for future reference will make sense to her), be firm in your dealings with the doctors; if this one doesn't have a clue, ask them to recommend someone who does see this kind of case, and go on up the chain.
When I had lupus years ago (a mild case, it turned out) I went to my family doctor for a year with no diagnosis before I gave up and went to a specialist, who on the very first visit told me what he thought it was, and the very next week told me the tests had confirmed it. This was after a solid YEAR with my GP. Made a believer out of me. There ARE people out there who see and treat whatever problem your daughter may have.
Good luck, and quickly. My prayers and the best of thoughts join those of others.
Ebbie