Does it make me bad mother if I don't want my ex husband's new gf around my son ? there is also a story behind this

no. you have every right as a parent to do that. if your ex can't accept it Oh well.

@ajsmith, no it doesn't make you a bad mom. But you have to think about it like this if y'all moved on and he's with someone else than your child is eventually going to be around that person you can't control that, vice versa. I know how you feel tho, I've felt that way before, but I grew out of that.

Not at all. You're his mother and you should have a say in who is around him. If it makes you understandably uncomfortable, they should respect that.