would you be pissed? what would you do?

i have bpd so sometimes i tend to jump the gun or think a little irrationally but im in therapy & on meds so it’s being treated. i was with my sons dad for 3 years. i miscarried in november. after the miscarriage he told me he was upset because once he got over the shock, he was excited so he wanted another. since i was so fertile after miscarrying, i got pregnant again right away. 2 days after our sons second birthday, i announced it to him and he basically kicked me to the curb. that was december 12th. so i grabbed my son & we moved out. on new year’s eve, he had our son and a group picture was posted of my ex, my son, some family members and a random girl i had never seen. i immediately knew my ex had invited this girl and brought her around our son. we had an agreement not to do this. he denied over and over & even gaslighted me telling me it was just a picture and it didn’t mean anything. i said okay & moved on. as time went on though, i just had a weird feeling he was lying. i finally got him to admit that he invited the girl. i still knew something was off so i reached out to the girl asking if this was the only time she had met my son. she informed me that they were dating and said she’s been staying over on the weekends and has met my son several times. she also had no idea we just moved out 2 weeks prior to meeting my ex nor that i’m pregnant. when i found out this information, i reached out to him and kinda lost it about how he’s been having this strange girl around my son for weeks and hid it from me because he knew we agreed to not do that. our son is 2 but has always slept with us. he does have his own bed at his dads but he never used it, not when we were living there atleast. so is this girl he’s known for not even a month sleeping in the same bed as my son? what happens when his dad has to shower? is he left alone with this girl? she’s around during diaper changes/bath time and all of these things are very intimate. im extremely uncomfortable with all of these possibilities. my ex has only known her for a month, so HE hardly even knows this girl. how in the hell am i supposed to trust her around my child? not to mention he broke a parenting boundary we had set (not in writing STUPIDLY). what would you do?