Exs wife freaks out on me YEARS later

Long story short, SIX years ago I dated this guy, we will call him brad.

I was single, as far as I knew brad was single, he didn’t wear a ring, I hung out with him and his friends, went to his place etc. We met at a bar where he was out with several of his friends and his friends girlfriends. We dated very casually for about 4 months, mainly just sex to be honest. I cut things off because I wanted something more serious and I met my now husband.

So now, literally 6 years later. I get a message on instagram from some woman, saying that I’m a homewrecker and she hopes my marriage fails, etc.

Turns out brad was married, and had 2 kids. I had NO idea. And I feel horrible. But she’s placing blame on me. I stopped replying to her after I let her know I had no idea that she existed and that it was years ago and if she’s mad at anyone it should be him.

I’m just confused as to why after 6 years she reaches out to the other woman, who literally had no idea. I honestly forgot that he even existed until this point and I don’t see what I did wrong as I thought he was single. Or if she didn’t know before, then why after years would he come clean about it but make it seem like it was my fault?