Severe gas pain immediately after and diarrhea x5 days post first time protected anal sex. ?

Doubt STD: The immediate symptoms were just irritation, gas, or something along those lines. No infection can cause symptoms that fast. Even the diarrhea probably is unrelated; most STDs from anal sex would cause rectal or anal pain, sores, discharge, etc. So probably just a coincidence. But see doctor if it continues. If STD risk (new or unsafe partner), rectal STD testing would be a good idea to be safe.