I Need Basic English Help?

I'm reading this novel and I don't understand what the author means when she says "thinking to bring it, a step ahead" This is where the sentence come from: The bag is biting into her fingers. She changes hands. The new Paula. Bringing a plastic bag. Thinking to bring it, a step ahead. She's rarely that skint that she can't afford the 15c bag tax. But she'd never have thought of it before, bringing the bag, before she left the house. Thanks so much.