Urgent!! Advise needed before I explode 😤

So my ex husband (still my husband but separated) and I been co parenting for almost the past 2 years. We had agreed that our oldest wouldn't meet his gf until we all sat together to discuss rules and boundaries. But my son just told me that his gf was at his grandma's house. I feel so offend and hurt. I feel like they are going behind my back so my son meets hers without my consent!

I'm not sure if I'm overacting but if we had an agreement I think we should stick to it. I don't think it's fair that they are trying to do this behind my back. I'm so upset at them and my ex mother in law. She knows how I feel about his gf. She's the reason why my husband and I got separated. She made me go through hell last year while I was expecting my baby. She told me that my ex husband was upset about me being pregnant. That's something I'm never going to forget.

Please give me some advise. I feel so angry and lost. All of this is still sometimes hard to process.

Oh and she recently had a baby with my ex. I kinda feel like she got pregnant on purpose so my ex husband would stay with her because in October they broke up and he came crying to me. I didn't take him back because she kept sending him messages that HE was hers and she was never going to leave him alone.

*** Update***

Let me add a some back story so it can make more sense

So late October (2017) my husband told me he wanted a break because he needed time for himself. I asked him if he has found someone else at school and he said no. He had recently enrolled to college for the first time. Within days, I found out he was seeing someone. When I asked him he lied about it. Until I told him that I knew about the girl.

He told the girl (who's the current gf) that we were in bad term for months, when we weren't. We were so good that we were saving up money to move out and had planned to take of my implanon in November to try for our second baby.

So lies were going both ways...

In January (2018), he told me that they had broke it off so we decided to see each again and that's when I got pregnant. But they weren't completely broken off. Well at least that's what the gf told me. After insulting me, calling me a bad mother and saying I got pregnant on purpose.

Months after that, were horrible. It was this back and forth between us, until I put a stop to it. I was emotionally tried to keep doing all of it.

My son is 10 years old and he had to get therapy to help him through this. We didn't officially tell him until August (2018) that we were separated and that his dad had a new gf. Since then my son has refused to meet her because in his eyes, she broke his home. But we did agree that if we all meet, then he would try to be around her.

Idk if this helps