Why would someone make up these lies?

[Lilly D]

I received a series of emails from a woman my fiance dated right before our wedding. She was telling me that he was cheating on me with her. She went on to say he cheated on her as well, once a cheated always a cheater. I wrote back telling her to back off and she wrote back going on about how when he broke up with her he just ended things and didnt communicate with her and even though she has now moved on it's a hurt she will never forget. THen she's like, bottom line he was in my bed that night.

My fiance denies that he has had anything to do with her. He said their relationship was brief, not serious and was basically a series of dates and yes they had sex. He said she became obsessed with him, would email, text, call all the time. She would show up at his work also trying to get him back. HE never told me about her. He was like, why would I tell you i dated someone like this would you have wanted to date me? He showed me some past emails from 2010/11 where she was begging for him back and he was telling her to move on. He said for the most part he didnt respond to her. One email was her telling him how she wasnt acting like herself, she had to get counselling, then realized she had thyroid issues and that's why she was acting so nuts. There was also an email from her to him asking about me and telling him she thought he'd end up with someone more glamourous than myself. There's also an email from her wishing him a life of misery...she also showed up at his work in 2012 which he did not tell me about!

All that aside...i dont know why I got brought into this. I am angry he didnt tell me about this, but i can kind of see why he didnt. He never thought she'd be emailing me. I just dont know why someone would make such things up. He said she obviously wants to ruin our relationship and does not want to see him happy with anyone else. This all happened 8 months ago and we never heard from her again. He says he hadnt even heard from her around the time she emailed me...I have never had anything like this happen. FOr a while i was ok..then this week i felt upset about it again. He gets mad when i bring it up now. He tells me it's over and done with, we've talked about it. I almost feel obsessed with her now. I want to know who this person is (she has nothing on social media). It's all so unsettling.
I cant imagine he would have cheated 7 weeks before our wedding. I was picturing myself cheating and I would feel so guilty. However i don't know if i'd be able to admit it. I probably wouldnt! I'd just act like it never happened and probably die on the inside a little bit each day. I just wonder IF he would tell the truth if he had cheated. He didnt tell me about her. Again i can see why he didnt...but it's still horrifying to me.
Thanks. He does laugh about it...but he gets up that I do not believe him. He dated her right before me which i think is a reason he didnt tell me. And i guess she is angry that he married the next person he dated. She is also 45...on the one hand it seems odd that she cant control herself at that age...but on the other hand she probably realizes that she's getting old and her chances of finding love are diminishing...at least with a man who has never been married like my now husband. Who knows. I cant rationalize it. My husband is not perfect, but he doesnt seem like someone who would cheat on me...especially 7 weeks before our nuptials and i was out planning. He would have to be a REAL maggot to do something like that. But i know things like that have happened in life to people before.
another thing that bugs me is that my husband works nights. She first mentioned he spend the night with her on a particular saturday (he was with me) but then in her next email it changed to friday. He was working friday night, not with me. Also she described what he was wearing as proof. It was kind of a generic outfit, jeans, plaid shirt, hoody, he DOES have outfits like that. He said, well i always wear clothes like that and she probably remembers.