Husband agreeing to childcare with ex

Now my husband and his ex wife have. children together and already have a custody plan through court. It's 50/50. Monday, then Wednesday, and then Friday, Saturday, Sunday, then we switch and their mom has those days. If that makes sense.

Recently I guess she's had a hard time getting a babysitter and my husband has been agreeing with her to take the children 100% which I didn't know about.

The reason this upsets me is because he works, I'm the one in charge of cooking, cleaning, giving them baths, taking to school, etc. I do absolutely everything, which I'm fine with. But if he doesn't tell me that I'm having to watch the kids on days I'm not supposed to, I don't have enough food (I buy groceries based on days I have them / days I don't). I also make doctor's appointments on days I don't have them because 4 children while going to an ultrasound alone? Yeah NOT!

I just hate that he doesn't talk to me about this when he knows I'm the one who cares for them, and this is now the 4th day in a row of me having the kids and I didn't know! I was so upset and he said "yeah I didn't want you to get mad" YES IM MAD! NOW I HAVE TO CANCEL MY APPOINTMENT BECAUSE THERES NO KIDS ALLOWED! The ex wife should be responsible for her kids on her days, but I'm the one calling off of work, cancelling appointments, etc because of this mess that everyone is keeping me out of. They just dump the kids the kids on me unexpectedly because I can't do anything once she drives away!