Should i tell my boyfriend?

I got really very drunk and kissed a guy back at the beginning of our 11 month relationship.. he knows this because i told him and we got through it and he trusts me again. However i got rung up the other day by my old best friend whose really close with the guy i kissed and he apparently told her we slept together and thought everyone knew which i really certainly didn't! I know i shouldn't have been that drunk in the first place so i don't drink anymore and i felt guilty enough about kissing him and now this.. I cant eat and barely sleep because i feel so guilty. Part of me says that he forgave me about it back then and technicaly this isnt a separate betrayal of trust, it happened over 10 months ago it would unnecessary to hurt him over this because i didn't know this happened until now so i wasn't hiding anything and i don't even know if its true. Then part of me thinks i have to tell him because it would be more untrustworthy not to. I'm quite sure he'd forgive me which is why i dont know if i should even bother telling him, but i'm not positive he will.. and i just don't know what to do? Also please don't lecture me on what a shitty girlfriend i was back then.. Everyone has to learn there lesson one day and thankfully iv got mine out the way.