Which drugs u r taking?
N sometimes, it may be due to mental or physical stress because pain is a subjective sensation. If all tests are normal, it means, you may have nothing wrong objectively. And the problem may be in the mind or some functional pain.
Anyway, looking to your investigations and history, you definitely don't have seronegative RA
Next Steps
physiotherapy may help a lot.
MRI KNEE joints can be done to rule out every single possibilities
Health Tips
consult any rheumatologist