My boyfriend's friend is spreading rumors about me for no reason.

Anonymous
My boyfriend's friend, Mark, never seems to dislike me when I see him in person. he is always really nice and says hi to me, but for some reason he seems to have a problem with me. I am a really good girlfriend and all of my boyfriend's other friends like me. But Mark is always calling him up telling him that he "knows for a fact" that I cheated on him, that I did this, that, etc. He told my boyfriend that I did coke, cheated on him, etc. Usually Mark simply heard this from someone else and reported it, but then he acted like he knew for a fact it was true even though it was only hearsay. My boyfriend said he didn't believe Mark, but it still bothers me that he seems so hell-bent on breaking us up. And Mark only likes really tall white girls, and I'm only 5'0" and Asian, so I know he's not just jealous or into me. What's going on?
Updates
+1 y
I should point out that my boyfriend won't let me confront Mark because Mark made me boyfriend promise that he didn't hear these rumors from him.
My boyfriend's friend is spreading rumors about me for no reason.
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