Could my boyfriend be cheating?

march 28-31, was on the phone with him while i was in shanghai while he was on his way to a club(which he hardly ever goes clubbin) with his friend whom was trying to hangout with a girl whom he had started talking to. since coming back from shanghai,me and my bf have been great. recently i checked his myspace and saw a comment on mar 31st from a girl named nikki saying" what up foo! had fun on friday" then after about a week...i checked again apr 3 she left another comment " let's get hazed tonight, call me so that i can come by and pick up a video, xoxo". (my bf smokes mj) but a few days ago i asked him if he had met or exchanged #'s with anyone at the club, he said no. i checked his phone while he was asleep, found a Nick(added as a guy/but for "Nikki"?) who had text similar to myspacecomments(w/othexoxo). i haven't mentioned to him about myspace. and he recently deleted the xoxo comment. if she's justanewsmokebuddy? why the cover up? bouttobookatriptocancunw/ him andnowthis? =O(