I dont know what to do and why does he care so much??? help please!!!?

Okay so, i have this friend who is actually my best guy friend( we will call him griff) and we have been just friends for quite some time until about four weeks ago we started doing things(not ) but we have kissed and messed around...well last friday we had a band banquet and i went with another guy and i didnt like him really at all, but he ended up telling me this sad story about his past relationships and i felt bad and we kinda made out, well my best guy friend Griff found out from one of his friends and he didnt tell me about it until we were talking about cheating and stuff and i ended up saying i kissed the other guy....well GRiff told me he was disappointed and he didnt say much about it, but he acting weird. we are not even in a relationship so i didnt cheat so i dont understand why he is so upset with me about this, but i do feel bad and i dont know if i should or not... HELP, PLEASE!!!!! i would be highly thankful:)