What is with this woman?

To be honesty it's a girl thing. We are very moody and we take things in the wrong way and once we found out an ex is dating someone else they feel cheated on. I have no idea why. I'm girl and I understand. Just best to tell her we are not dating and I am able to see other people as you are too. She will probably go cry later on but she shouldn't be talking to you for a while.