I'm trying to be strong!!

I have been NC for about 4 months now. We share a daughter so there is communication at that level...but only through a text message and I get straight to the point when I need to say something. I have truly NEVER gone this long before without "caving in" and running back to him. I am really proud of myself so far. I thought I was doing GREAT!!...UNTIL YESTERDAY. My daughter came home from spending the night with him. We talked about how her day went and what they did together...she mentioned to me that her & her dad went for a ride and he told her that he has been talking to another "girl". And then proceeded to tell her, "But don't tell your mother".
I don't get it! Why would you say something like that to a 12 year old? He is 52 years old! Does she really need to know his personal life? Or does he just want her to come back and tell me? AND WHY? UUGGHH!!
Anyway, it has been weighing heavily on my mind about this other woman. I am using every ounce of will power inside of me not to call or text him. (which in the past i would have probably already done) I feel really weak today...and I don't know what to do.