Why would a person ignore someone when they say they're having with your significant other?

Well me and my babys father broke up and he hooked back up with the mother of his older kids and they live together and have been for a few months now. Well me and him still have from time to time (judge me if you want) and when I got mad at him about a month 1/2 ago I told her that we still have and that he sees me and she never wrote me back. I wrote her on facebook. Why do u think she didnt write back? and I dont think she told him anything because he never mentioned me saying anything to her...