How do I talk to this girl about what happened?

This girl have been hanging out with and seeing exclusively for 2 months hungout with her ex and I got mad at her for it because she was secretive about it but she said if I asked she would have told me because I knew he had come by the city... So we didn't talk after that... but a week later we started kissing and seeing each other at random places.. she doesn't text me as much, but will respond to me... I want to ask her about what happened with her ex and where we stand... but idk when or how to bring it up? If she did something with him, or is still unsure.. I just want to know so I can move on.