It’s happened to me. Just the worst. I found out it had nothing to do with me, a while after from a mutual friend. Turns out I had been “the other guy.” I even briefly met her ex, who she went back with. I was used, basically, to make him jealous. I don’t know whether that was intentional or not. But it was truly weird, for me, the way it ended with no explanation. I doubt it was even intentional on her part. She liked and was attracted to me. But she was in love and attached to him. She acted in a way entirely consistent with that. I just didn’t know it at the time.