What could be likely causes of sudden severe elbow joint pain, beginning in elbow, spreading to every joint, extreme fatigue, low fever, for weeks. starting day 3 of doxycycline to treat chlamydia sexual transmitted. coincidence? or same problem?

A drug reaction is possible, but you need a full evaluation with bloodwork in person to get to the root cause.

This may not be a coincidence. Chlamydia itself doesn't cause arthritis, but it can trigger a type of joint inflammation called reactive arthritis. Another possibility is that you also had gonorrhea at the same time. It often accompanies chlamydia and can cause disseminated gonococcal infection (or DGI) with symptoms like these. See an infectious diseases specialist or rheumatologist ASAP.