Sex guilty (long story) help ASAP

Ok so my bff has a super hot brother that I have always had a crush on and last week when u was suppose to pick her up to go to the movies she had an errant to run and would take a several hrs. Her brother Moises kept me company. We were sitting on his bed playing video games when I dropped my controller and bent over to pick it up and caught him checking my but out thing is he has a gf, so we stopped playing after it got awkward and he said that he and his gf broke up bc she cheated on him again. I hugged him as a sympathetic friend but he caught me off guard and kissed me I pushed him away but ended up kissing him again we made out for a few minutes and one thing led to another and we had sex we did use the morning after pill and a condom. My problem is we have been secretly dating for a week and I feel so guilty what should I do should I tell my bff?????