The symptoms of STDs include warts, ulcers, yellow or white vaginal/penile discharge, burning with urination, pelvic
pain, bleeding between periods, testicular
pain and joint pain etc. You do not seem to have any of these symptoms. These symptoms which respond to an antihistamine are probably hives. If the symptoms are severe and persistent seek the consultation of an allergist for further evaluation and management. Do keep me posted.
Best luck and regards!