Confusion while working at my retail job

The other day while working at my second job, I didn't really understand something. I work as a cashier and my customer had two separate orders. I was fine with that but then she handed me some coupons. I thought that the coupons were for both of her orders. When I told her I'm sorry but I don't understand and she repeated it again. I think that I told her that it wasn't her, but that sometimes my brain doesn't work right. She seemed understanding and said that it was okay. When I started her second order, she handed me the coupons for that order, so everything was fine. I really hate when I get confused!