Hi I had anal sex with man, me being the top and him being the bottom. He told me he was on prep and he told me how it prevents the chances of catching Hiv. So I had unprotected sex with him. Am i at risk since i wasnt on it and he was?
Pre-exposure prohylaxis, aka "PrEP", is a prescribed regimen that reduces one's chances of acquiring HIV by over 99% (http://www.natap.org/2016/ICAAC/ICAAC_02.htm). This is considered "protected sex" by the U.S. CDC and many other medical organizations, given it nearly eliminates risk of HIV transmission. If someone does not have HIV, they cannot transmit HIV.
In other words, if your partner was using PrEP as directed, then you did have protected sex. It may not have involved latex barriers, but it was nevertheless protected by the barriers that PrEP creates in his own body.
I understand this goes counter-intuitive to the 36+ years of HIV prevention messaging that has been prevalent in most parts of the world. But fortunately, we now have options to prevent HIV that don't depend solely on condoms being used for anal sex. PrEP, and Treatment as Protection (aka "TasP") are both effective ways to successfully prevent HIV transmission whether condoms are used or not.
Now let's say for the sake of argument that your partner wasn't on PrEP, and was living with HIV with a detectable (transmittable) viral load. Even in that scenario, your chances of acquiring HIV as the top from this one encounter were still less than 1% (https://www.poz.com/article/HIV-risk-25382-5829).
When we put these pieces all together, we can see you were not at risk of acquiring HIV by topping a person on PrEP. But I would not advise you or anyone to give another person the power to determine your sexual decisions. If you enjoy condomless fucking with other guys (as many of us do), perhaps you'll evaluate if PrEP is right for you. Here is a link that discusses more about PrEP (http://www.thebody.com/index/treat/tenofovir_prevention.html) as well as global resources for accessing PrEP (https://www.pleaseprepme.global/).