M.D.
There's an old saying: If someone takes advantage of you once, shame on them. If they do it twice, shame on you.
I can understand why you don't want this to continue, but you should have put a stop to it long ago if you weren't happy with the arrangement. Don't be resentful. She didn't put you in this position, you did by not speaking up as soon as you realized you weren't happy with the situation. Take it as a lesson learned and don't offer to help her out again.
As for how to politely tell her that she still owes you for last week, I would say "I know you have a lot going on, but I'd like to remind you that you still owe me for last week. Please bring the $20 tomorrow. Thanks."