What is wrong with him?

Me and my ex have been broken up for about a month now. We were together for a little bit over a year. When I broke up with him he tried to get my back and said how he loved me so much, sent me flowers and a teddy bear and stuff like that. While going out I always got this sense that he wasn't honest all the time (I won't go into details) and my family kept telling me how he seemed untrustworthy. Anyways, just a few days after I broke up with him he asked me to think about what I did and see if I am happier w/o him. I said ok and then we decided not to date other people in the mean time. Long story short, he met some other girl 2 days later and started flirting with her (dont' worry about how I know, but these are true facts) and so I called him and told him I was happier w/o him. and I am, I don't miss him. but after just 2 weeks she became his gf and she's saying how she thanks god for him and all this stuff. so, my question is. is he just doing the rebound thing? or did he cheat on me while we were together and never was honest with me?? did he lie about loving me??