The causes are infections, medications, bee/wasp stings, food allergies, environmental allergies, stress, cold temperatures etc. Sometimes the causes are idiopathic. 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.