Danny sounds to me that this was after, she found out what he did with her friend.
Danny, think about one thing, loved, jespah's reply to you.
If a woman has an ex, and that ex, sleeps with a friend when you are over, you are at your very lowest, wondering if you were ever good enough, wondering if you were pretty enough, because he did that..
It's like a grief.
That is why she did what she did, to her it wasn't wrong ,that was her ex. Her time with you was short, with him 17 months. She felt rejected again..
Be friends, remain a little reserved, live your life... Until she can work out what a piece of work he is, for doing that, through perhaps his own jealousy.. and that she is worth more... she needs this time.
What she needs to see is he is a cad and you were the better guy for her.
But whether you are willing to accept that, IDK.,
My honest advice to you is, never enter someones life that has baggage , ie) still getting over someone, as usually something like this always happens.. As their emotions are playing on them, they want the new, but haven't gotten over the old and if the old turn up deliberately? Just to peev you off? Then he/she will win.