What would you do in this situation?

So last night I'm hanging out with a friend who i've known my whole life but only see maybe every couple years hes my god-mothers grandson. Anyway we decided to go to the little bar in town have a couple drinks and catch up on the last 3 years. I have an amazing boyfriend who i absolutely love with my entire heart he is everything to me. That being said we had gotten into this ridiculous fight one of those little things that just get blown up a couple days ago haven't really seen each other giving it some space to cool off. As i was leaving my friend out of no where kisses me. At first there was a little shock and a small miniscule amount of interest and then absolute shame. I stopped it and it means nothing to me how could it im miserable sick to my stomach today over it. Here's the problem do i tell him and hurt him and possibly lose him over something that was a mistake and will not ever happen again i have never cheated and have no interest in cheated. Or not tell him and hope i can get over the guilt if not its mine to bare anyway. I know i'd want him to tell me if it were the other way i really would that's why im so confused i would just be devistated if i lost him over this or he never looked at me the same. Any advice and please don't be mean to me im really really upset thank you.