Physiotherapist
You can try to bring your concern to this doctotr again, or you can always see someone else for 2nd opinion. Though this is not to say that X-ray will be useful or help making crucial diagnosis in your case.
If you got complicated medical history, speak to GPs. Different doctors have different approaches, and some communicate better than others.
If your definition of 'very fit' means free of any significant illness, you can speak to a decent physio first without a referral. Ask your physio to communicate with your doctor with any findings if he fails to fix / improve your problem.
It can be stress, degeneration, postural, musculo-skeletal issue or something else. Each professional can only try to diagnose you in the area of their own speacialty, so information sharing between physio, doctors and specialists is important.
You will get the answer you seek, don't stress
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