I just recently had anal sex where I was the bottom and I didnt realize the condom had broke until the top pulled out. but by then he had already cum. I'm not sure if the top is pos or neg but he claims to be neg (last tested 4 months ago). What are the risks of contracting hiv if lets say he is hiv positive? Its been like 3 days so I think its too late to use PEP ? (i didnt know about PEP until now)
The CDC estimates the per-act risk for acquiring HIV from unprotected (or broken-condom) receptive anal sex with a partner confirmed to be HIV positive is 5 per 1,000 exposures. Of course there are many confounding variables that can affect HIV transmission. These include both viral (viral strain, viral load, etc.) and host (immune integrity, genetics, concurrent infection, local trauma, etc.) variables.
PEP is most effective when started as soon as possible after an HIV exposure. If greater than 72 hours have elapsed since the exposure, PEP is no longer felt to be effective and therefore it would not be warranted.
I would suggest you get an HIV-antibody test at the three-month mark. Also, if you are still in touch with your top gun guy, you could ask him to get retested. If he's negative, that would significantly decrease your HIV risk. He might still be within his "window period" if recently infected and therefore test HIV negative, but the odds of this being the case would be remote.
You can learn more about HIV-transmission risk, PEP and related issues in the archives of this forum. Have a look around.