She's not sticking to the custody schedule

My ex keeps texting me last minute saying she needs to switch weeks because of her work. First time I was cool with it but now it's becoming a pattern and I feel like I'm the one adjusting my life while she does whatever. I don't want to be the bad guy but I need some stability too. The kids are starting to ask me why their schedule keeps changing. Should I push back harder or am I being petty about this?