Concerned about my marriage...

So this girl that used to be my friend and I lived with for maybe a month gets a job working with my husband. Well I found out that about a month ago she had asked my husband how our marriage was on two different occasions. Well Friday he tells me that she come to him at work and was telling him all kind of stuff that happened in my PAST before we got together. And she also accused me of sleeping around on my husband. She is constantly telling my husband all of these things that I done before we got together. I confronted her today and she pretty much told me that she hasn't been saying anything that my husband was making up that stuff. My husband never knew I did any of the stuff that he came home and asked me if I had done. He even told me that she had been asking some guys at work where he was at when she didn't see him this morning at wrong. My husband and I have been married for almost a year now. Why is this girl bringin my past up now? She made my husband promise that he wouldn't tell me that she was saying all of that stuff but of course he told me anyways. I just don't understand why this girl is trying to ruin my marriage over stuff that happened before my husband and I got together. I don't know if she's doing it to be spiteful and hateful or if she is trying to get with my husband. The bad thing about it is that she's in a relationship and they've been together longer than my husband and I have been. And he's following her footsteps. Why are they trying to ruin my marriage? I don't want her to get to my husbands head and play with him and make him think that I've done that stuff while we've been married because I haven't. Our marriage will never succeed if she is constantly telling my husband negative things that I done in my past. I need help please!!! What do I do in this situation??