I don't know if you feel this way, but sometimes I'm so sick of having to change shifts with coworkers who are out because of their period pain.
Well, sometimes we all don't feel well and need time off, why does it bother you so much?
It gets really annoying to have my schedule set and they I have to change it for one or two people.
You don't seem to think "period pain" is real. It's rude to judge pain someone's pain level. It shows kindness when you can help your coworker out.
I think many of them are taking advantage of that excuse just to switch swifts they don't like.
Be more accommodating to your coworkers. You don't know how much pain they are going through at the moment. I am sure they appreciate the help.