GUYS what would you do if ur girlfriend did this?! help!?

i was at a party recently and got very drunk... i was chilling with a bunch of people in a hot tub and then next thing i knew some guy started kissing me and it took a second for me to be like UHMM NO NOT OK.. i love my boyfriend more than anything in my life and i wouldn't ever do anything to intentionally hurt him. we've been going out for over a year. i've been with him once since this kiss happened and i didn't tell him... but the whole time i was with him it bugged me because he is the best guy ever to me. i feel like not telling him would be unfair to him but i feel like telling him would make him feel very hurt... i just don't know what to do.. please help:(