Bottoming is when your guy puts his penis in your butthole. This is a higher risk activity than topping, or putting your penis in your guy's butthole. This is because the sensitive skin in the butt can be easily torn and is very susceptible to HIV and other STDs.
HIV-negative bottoms are at risk of picking up HIV if you are having sex with someone who is HIV-positive or whose HIV status you don't know. The risk of infection increases if the top partner cums in you without a condom. The risk also increases if the HIV-positive top is newly infected, not on treatment, or has another STD like chlamydia, gonorrhea or syphilis.
Condoms are the best way to prevent HIV, even if your guy has an undetectable viral load. If your guy is HIV-positive, taking his medication as directed and getting tested for other STDs will reduce the chance you become HIV-positive from doing it without a condom.
It is possible for HIV-positive bottoms to pass HIV to a negative top, especially if they are newly infected, don't know their status or are not on treatment. Both negative and positive bottoms are at risk of getting or passing other STDs, like gonorrhea and syphilis.
Taking medication as directed by your doctor is an important factor in keeping your viral load undetectable. It is also important to get checked regularly, in your butthole and throat, to make sure there is no other STD that could be increasing your viral load.