The pain can be due to reactive arthritis. One thing you should notice is that when the elbow
pain comes—does the
knee pain decrease? If yes, then this is migratory polyarthritis and this can be due to Hepatitis B, Hepatitis C, rheumatic fever, SLE, Lyme’s disease, sarcoidosis, Whipple’s disease, Henoch-Schnlein Purpura, and bacterial endocarditis. Hashimotos thyroidits is another possibility. It can also be fibromyalgia and or chronic fatigue syndrome.