Time to explain that you were new and had no understanding of pricing.
I'd tell her that due to your regular customers, you only have a few days per month where you can do one off jobs and these are done for regulars.
Tell her you will give her a quote for the roof, but its under a new pricing scheme from when you last cleaned it.
Be nice and a little appolagetic and you'll probably still get it. She knows you do a good job, that's why she's rang you.