One theory I have is that there was a PA 7 years ago.
It was at the funeral of an ex bf's grandfather.
Maybe a one time fling.
Shortly after that I was joking about something and she flew off the handle thinking I was accusing her of infidelity.
That's when our problems started.

She came home that night and there was something wrong but we had mind blowing sex that night. Since our sex life was nearly non-existent at the time it blew me away but I always wondered what sparked it. When we went to MC later she claimed she felt she was still in love with him but later said that was a mistake.