I would suspect a rheumatology issue. Your symptoms don't happen without something going on. The fact that it is bilateral instead of unilateral makes me think you are having an autoimmune response to something.
I'd just like to suggest that you speak with the doctor who ordered those tests that came back ok. Tell them that while you now know what's not wrong, you still need to know what is wrong, and what other possibilities there could be and what is the next step to finding out. If they say they don't know, ask them what kind of specialist would be appropriate for you to see. It would be hard to even venture a guess without seeing what you describe. Hopefully your doctor will be able to satisfactorily address your concerns.