What are the chances of testing hiv negative 7.5 weeks after possible exposure and still be infected?

I'm a healthy 20 yr old male. i tested negative nearly 8 weeks after but was wondering what the chances are of still being hiv infected. My guess was that the hiv antibodies would have built up by now to show up in the test. also, i didn't even have , it was just unprotected . only problem is i didnt know the girl and that was the first and last time i've ever seen her (it was at a college party). so with that said, should i be worried?