So me and the father of my son aren't together anymore and today he took the baby to some girls…

So me and the father of my son aren't together anymore and today he took the baby to some girls house without letting me know. I personally feel uncomfortable because I don't know her. I've never met her. Ever. And he didn't even tell me that he went to see her, I had to find out through pictures she took of my son with her son. Am I wrong for being upset? Am I overreacting?