Just ask for an std screening and put your mind at ease. Don't put it off any longer or else your mental health will only get worse about something you may not even have. and if you do, (which I doubt) there are worse things in the world. I have sarcoid, (rare disease) didn't ask for it, didn't want it, didn't contract it, but my body/god decided to give it to me, now I just gotta live with it.
Im a little confused... You said you were tested? and it was negative? if so what is there to worry about? I'm also willing to bet the girl was joking... What girl is going to come out and say that and risk her reputation at the highschool/college level? If it makes you feel better get tested again.