If you look at this from your wife's perspective - she walks in and sees you kissing your ex, and then guiltily pushing her away when discovered, you can't really blame her for being shocked and upset.
Sit down with her, look her straight in the eyes, tell her what happened, and that you love her, and she knows that.
Women are experts at spotting a man who lies - she needs to be reassured.
Oh, and next time your ex is within touching distance, remove yourself from the room, quick!