Is it ok for me to have a problem with my ex partner introducing his new girlfriend to my child?

Let me clear something from the beginning I do not have any feeling towards my ex but I have an issue of him bringing my 2 year old boy to his new girlfriend. There are lots of things that to me seem weird after each visit that my child takes to his place. I have raised my consern to him of what I feel uncomfortable and he just dont want to hear it.. He says that he does not have to share what he does and goes on with our son when he is with him. To some extend i understand that and respect that but when my son comes crying saying he does not want to go back to daddy's and i heard him telling a littler girl do not hit me over the  phone  makes me wonder if he should have an open door policy when it comes to spending time with our son.  I have a problem with her changing my sons diaper and intruding with the uprising of my child specially that they have only a month or two together. I believe its way to fast. Am I wrong with feeling the way I feel ???