I'm sorry that you're so scared, but stomach aches and diarrhea aren't symptoms of STDs.
You could just go get tested and put your mind at ease, but I can't see any way of being anally raped/penetrated without any physical signs of it soon after.
Also, for future reference, if you're interested, there are ways to make anal sex not hurt. It's still not something you wouldn't have any signs of the next day, though.