Stuck in the middle?

i had a gathering of my close friends at mine when my parents went on holiday. i couldnt remember much of that night except that i was sick by drinking coconut malibu (yuck). apparently the next morning my boyfriend told me what my best friend mel was doing at my gathering, she was kissing my long time friends boyfriend and another one of my mates. then he said she kissed him on the cheek... it didnt bother me as much but recently i cant stop thinking about it so i messaged her on myspace to see what happened that night and she said my boyfriend kissed her when i told my boyfriend that i messaged her he went mad asking why i'm bringing it up. i asked him again what happened and this time he said she kissed him on the lips :S what the hell? i really dunno who to believe, me my boyfriend and my close friend have known each other from college for one year and my boyfriend did fancy her before he liked me. im just upset that one of them is lying.