I applied for a nursing degree at uni (didn't get in :() and it was made very clear that shift work could only be rearranged in extreme circumstances. For this reason my ex is not plan a, or even plan b for childcare.
That said, there needs to be a little give and take. Ex is self employed, and a couple of weeks ago got some work in on the Friday, to be done on the Sunday, which is his contact day. I switched days. I had to cancel the plans we had, but no major biggie.
As the parent with care (although I am meant to have my dc 11 out of 14 days, but realistically I have them 12out of 14 most if the time) I don't have a social life, or a partner because I have the dc. That doesn't mean to say I shouldnt have, but that's the reality, and if were to ask anyone to have them so I could have a regular social life/relationship most people would decline.
I've also had problems with ds1 resulting from the fact his dad is in a new relationship, and never spends time with him without the gf and her dc being there, and he has on occasion cancelled contact to spend time with her, making a child feel they are second best is very very IMO. I'm not saying that's what op is doing, its just my thought.
So, aybu? I don't know tbh. I can see both sides. A bit of flexibility on your side should be there, although not so you can have 'me time' or a gf if I'm honest. But your ex would have known in advance that she would be working shifts, and should IMO have made sure she had childcare in place (by which I mean you being flexible)