Maybe my ex is in a rebound?

My ex and I broke up 2 months ago. He found a rebound a few weeks after. She is sleeping next to him at night apparently. She express that he is constantly lying to her. But when me and him were together, he didn’t lie to me and I didn’t have to worry about other women. Did he move on or is he using this to not feel hurt. We were together for almost 4 yrs.