13 year olds really do not have STD's-for the most part. I would just see a Dr about the irritation/rash.
The other person was 13 too.
Ok you had sex when you were 13? How old was the other person?
Chances are you did not get an STD when you were 13 years old. Best thing to do is to see a Dr and let him/her examine your rash. Chances are it is fungial infection, but like I said only a Dr would know.