Your rash sounds like irritation to me, not an STD. Did your doctor say what the antibiotics were for? Maybe you should consult a doctor skilled in STD diagnosis, like a dermatologist or an STD clinic. That's the best way to get a diagnosis and alleviate your paranoia. Avoiding unprotected sex might also be a good idea, even though it doesn't sound as though you have an STD now. Remember not to pick or rub the area, so any doctor you see will be able to tell what's going on.
Best.
Dr. Rockoff