My (23F) boyfriend’s (22M) best friend (23F) spreads rumors that I cheat on him

My boyfriend Sam’s best friends with this (usually) extremely kind hearted girl named Rosa.
Now, Sam and I have been together longer than they have been friends. We were high school sweethearts and he met Rosa in our college, she had been doing the same major as him.
In our senior year of college, last year, I went on three dates with this guy, Allen, when Sam and I briefly broke up.
Now Rosa started going out with Allen most of last winter until now, but sometime last month Allen responded in a creepy way to a booby picture of me on my private instagram. He talked about how beautiful I was and that he had a girlfriend (Rosa) and joked about how he was ready to leave her for me. Needless to say I removed him from my followers.
Rosa broke up with Allen and now for some reason she’s taken to telling everyone who knows me that Allen cheated on her with me, and that I cheated on Sam.
I confronted her and she essentially told me to remove myself from her life without really addressing what I asked her about.
So I consulted with another friend of ours and this friend said that Rosa has had feelings for Sam for the longest time.
I don’t know how to feel about this. Sam doesn’t know any part of this. He doesn’t know she has feelings for him and he doesn’t know about her spreading rumors about me.
I’m not sure how to act. Should I tell Sam? I kind of don’t want him to hang out with her as often but I don’t know if it’s reasonable for me to ask him of that. I can’t have her saying these things about me. I really am at a loss for any ideas of what to do.
TL;DR: My boyfriend’s best friend has feelings for him and tells everyone her own ex boyfriend cheated on her with me. Making me a cheater too. That’s not true.
EDIT: I want to clarify that I had already told Sam about Allen’s creepy message the moment it happened. And he knows I went on a few dates with him while we were broken up.
UPDATE: I told Sam about what Rosa did, and he was mostly shocked that she would behave like this randomly. What she did though, was tell Sam that I had sent Allen the photo, making it sound like I was deliberately trying to seduce him, and that Allen had rejected me, which is NOT what happened AT ALL. I told Sam not to talk to her about me anymore and that she was lying but he told me not to be “meanâ€. I asked him to put some space between them but he said she just needed help and that he was her friend and was going to be there for him. I didn’t tell him she had feelings for him though because in the context it would have sounded like I was desperately jealous and looking for reasons to end their friendship. Instead he said he wouldn’t let her bring me up again when he was with friends, and he would cease to talk about me to her. It doesn’t feel like enough. He’s not as offended as I am about the lies.


Source.