Ex wants to switch weekends last minute AGAIN

This is becoming a pattern and I'm losing it. My ex asks to swap weekends literally the day before, and when I say no bc I already made plans with my kids, she acts like I'm being difficult. Last Saturday she texted at 8pm asking if she could take them Sunday instead of Friday. I already had tickets to the science museum booked!

I'm not trying to be rigid about the schedule, but there's a difference between flexibility and being a doormat. The custody agreement clearly states Friday-Sunday is my time. Am I crazy for wanting her to give more notice?