My boyfriend makes "dates" and plans with a girl i'm suspicious of secretly but i know. what do i do?

i have my boyfriend's myspace pword and he doesn't know. he thinks i've forgotten it. but i found him making plans and "scarface dates" with this girl and he wants to hang out with her all the time but he tells me he doesn't want to see me all the time.. and i've been suspicious about her for awhile and he says i have nothing to worry about and he would never like her and they're just friends. i know she likes him tho. he has never given me a reason to not trust him but i'm still jealous.what do you think i should do without him finding out i have his pword and do you think that he's trying to get with this girl?