After i told him hes acting really weird! help!?

ok so i told my boyfriend who i have been with for a long time now about his best friend. all my mans friends love me and say im the best gf he has ever had but they always make comments about if we break up i can cry on their shoulder, or they will comfort me. its just fun and games but one night his friend "accidentally" brushed his hand across my backside, and then did the same thing to my chest. so i felt really uncomfortable and told my boyfriend about it and that i felt really uncomfortable around him. he told his other friends about what i told him and they said he should believe me because none of them think i would lie or exaggerate something like this. so he said he believed me and he would talk to him about it. all of a sudden he is acting a little wierd. he seems not as happy sometimes and hes a little distant. it was his best friend so i know hes upset about it, but i dont know why hes acting wierd towards me. also the other day when it was his group of friends me and my friend he sat me in between my friend and him. im thinking he did this for me, but i cant tell. help!