Is my reaction justified?

BM messes us about a lot with times to collect their son. She will consistently tell us one time then change it a few hours before we are due to collect him. She is off work for a few days and we were meant to collect SS from nursery as per BMs schedule she sends my partner each month. He messaged today just to double check this and she replied we are to collect him from hers at 7, if he hadn’t messaged she wouldn’t have said anything and we would go to collect him to find him gone. My partner changed his shift so he could collect SS and he dropped me off at work so he had the car. Now I’ve got my mum taking me home as she works in the building beside mine, he’s lost a day of work and wages and he has less time with his son as when we collect him he generally goes to bed 30 mins later. Normally I say nothing but my partner uses my car and fuel I pay for, she constantly does it and messes up our plans. I honestly have had enough and my partner said ‘don’t worry, it’s done now’ but I know for a fact of I, or his other sons mum, did this he would have kittens! I’ve told him I will no longer be collecting or dropping off his son after October, as we already have arranged days so he can work, and he will need to buy the back seats for his car so he can get a car seat in there, he has a vw corrado with the back seats out. He told me I’m making something out of nothing but it’s really gotten to me how much he lets her get away with. I don’t get along with her but he does as well as her family and constantly does loads for them but nothing for me. He will drop everything for them but I ask him for months to help me with stuff and it never gets done!