How recently did you get with this girlfriend of yours?? That's where I would start..
Also, your 2 y.o. is at a pivotal age.. and doesn't quite understand the world.. He knows that his mommy loves him and takes care of him..
Does your girlfriend do the same (she should.. anyone you get together with should love your child as her own.. ) The problem is that at his age.. has he called your gf "mommy"..
His mom/ your ex.. will get extremely upset by this sort of thing.. there is no way around it.. just through it.. Make it clear to him in nice soft words who his mommy is.. and if the gf is close enough, he could have a pet name for her.. but nothing close to mommy..
As far as the legal part of it, I don't know about your state, but in many.. If you call the police and say that she has broken the order.. They could arrest her.. Now, they most likely won't, but will insist that she follow the order.
You probably don't want to go that route, but just tell her that she is breaking a legal document, and that is considered contempt and she could go to jail for such a thing (though I wouldn't make it harsh because if you piss her off.. she'll come back and slap you in the face with it.. like if you are 5 minutes late bringing him back..)
Really, the best thing to do is to be nice.. Talk to her kindly.. Don't get frustrated or upset with her.. I know you will probably feel that way, but don't let it show.. Bite your tongue..
Explain to her that you want to work things out..
You don't have to be best friends, but you should be able to get along for your son's sake.. After all, you will have to put up with each other for the rest of his natural life.. and if you both out live him.. then longer..
Why do I say this.. a lot of people will say until he turns of age..
Well.. You will have to see her at his graduation from high school, his graduation from college, or the military or what have you, you will see her when he gets married, and how many other events over the course of his life..
You better start getting along now.. It will make things soooooo much easier..
Good luck.. and if it's hard to keep your cool.. Force yourself to smile.. It's actually fairly hard to be mean, when you are smiling..