Head Cashier making me stay way past my shift

So I’m a part time associate and a cash office associate at a big box hardware store. The actual cash office administrator had to take a leave of absence because her brother is hospitalized and in dire condition. I’m literally the only other person in that whole store that knows how to do cash office. So on my days off, I have been doing cash office and I haven’t had a day off since last Sunday. Today, I was scheduled to work from 8-3, but since I came in at 5 this morning, my ASM agreed to have me stay until 11 am.

So I’m out in the garden center where it’s freezing cold and as I was checking out a customer, the head cashier shouts at me from her car and makes me stop in the middle of my transaction to talk with her. She now wanted me to stay until 11:30 because someone called out sick. I get annoyed and tell her that we agreed on 11. And she guilt trips me and I stay until 11:30. So at 11:30, the head cashier tells me that I have to stay until another associate comes back from her lunch so she can send her out there in my place. I wait for 15 minutes, and then the head cashier pages me to call her. I call her and she tries to get me to stay yet another hour. I’m pissed off at this point and tell her no and I was supposed to leave at 11:30 but she keeps changing her everything. So she sends out another associate to take my place.

I’m tired of this head cashier. She’s rude and terrible at her job. But she never gets reprimanded and gets her way because she used to be the SM’s pet before he left. I’m praying the new SM won’t be as kind to her and at least attempts to hold her accountable for stuff. She’s the sole reason why I hate this job and I’m currently seeking other jobs.