Advice please

I am a supervisor at my job. We are extremely short staffed right now and we all have been getting a lot of over time. We are all tired from working open to close everyday.

It was a little slow at the end of the day, so I told one of the employees she could leave leave a half hour early. (Most of us would be happy about that).

She didn’t end up leaving early, so I told her she could come in tomorrow 30 mins later.

My boss wants to try to get rid of a little overtime for all of us.

This employee started crying and asking why we were targeting her and why she’s the one we wanted to leave.

I don’t understand why she acted that way. We try to let people leave early all the time to shave off some overtime. My boss lets me leave early some days … it’s not like it’s something unusual.

So she threw a fit and cried as she left today.

I don’t think I did anything wrong. I don’t understand. Any advice ??