3 weeks after sex I had a rash, swollen glands, night sweats, weight loss but tested negative for stds & hiv. What caused my symtoms? Cbc is abnormal.

Mononucleosis. Sounds like Mononucleosis Syndrome.
Differential is EBV,CMV, Hepatitis and other viruses.
Best follow up with your PCP or go to ID Specialist for further evaluation !
EBV? CMV? Unrelated? Most likely just coincidence, unrelated to the sexual exposure. But possibly Epstein Barr virus (mononucleosis) or cytomegalovirus (CMV), but you’re older than average for both of these. In any case, probably nothing serious. But for maximum security, play it safe and be retested after ~6 weeks for HIV and syphilis.