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There are plenty of sources on the internet about the connection between joint pain or stiffness and allergies. Keep in mind that you should seek medical advice and not try to diagnose yourself. But if doctors can't find out what's wrong or treatment doesn't help you, consider the possibility of allergies.
And the fact is, eating well may not cure what ails you, but it's never going to hurt. Eating nourishing food while avoiding foods or substances that may cause problems for you is cheap and non-invasive.