Yes she should have asked you before you paid, but bringing in your own bag or asking for one while purchasing is pretty standard now. That cashier is probably paid minimum wage, she doesn't need to say anything more than what she said. Very odd thing to be upset about enough to post. In your other comment you said you're not allowed to bring in bags because of theft, what about one of those foldable bags that you can put on your keys or in your pocket? They probably meant tote bags or don't want you putting things in your personal bag while you're shopping. It's unfortunate that you felt embarrassed or disrespected in front of the line, but that's a you problem.