During that time, it is important to refrain from having condomless sex with partners who are HIV negative. If this happened more than 72 hours ago, your best bet is to get tested in about 14-28 days (depending on the kind of test you are given).
If this potential exposure to detectable HIV occurred within 72 hours, then PEP (Post-Exposure Prophylaxis) may be recommended. Some people speculate that immediately pushing his cum out of your bum after sex may decrease the likelihood of HIV transmission, but I have yet to see any research to corroborate this theory. Receptive anal sex is the most common way HIV is transmitted, and defecating afterward would not minimize that risk. If your boyfriend has a detectable level of HIV in his system and if you're not on PrEP, then you would be at some risk with his cum in your bum. If your partner had a detectable viral load, but you were taking PrEP, then your chances of getting HIV are nearly zero. PrEP is a daily pill that prevents HIV by more than 99% (). You also don't mention if you are taking PrEP to prevent HIV. Even if they are undetectable for less than six months, their chances of passing on HIV to their partners are significantly decreased.
If someone is HIV+, but undetectable for six months or longer, they cannot transmit their virus to others ().
