Ahh crap. I feel kind of bad.

I guess I'm in the minority with this one.

My vote is to leave it. Technically, she did not pay. She left you positive, that in no way means you are obligated to leave her one or you are obligated to not leave a negative.

If your conscience eats at you, have it removed... but honestly try to reason with yourself first.
 
I would leave it. Paying after 22 days is unnacceptable. I probably would have left her neg FB before 22 days anyway. If you want to make a comment you can add "finally paid after 22 days" or similar. Don't feel bad, other sellers need to be warned.