How should I bring up a pay raise with this person?

I am a freelance writer. In December, a woman hired me to write a 20-page report for her, and we agreed on a price-per-page that I thought was reasonable. By the time I completed the project, my feelings had changed. Looking back, I think she got a total bargain out of the deal.

She has requested my services again and wants me to write 3–5 pages by tomorrow evening. We haven’t discussed my payment, but I think she believes that I am content with the rate she payed before. I’m not. I don’t know how to say this to her tactfully. Given the short deadline, I think I would rather her pay twice as much as before. (She gave me advance notice that she would need this work done about two weeks ago, but I haven’t been able to work on it until yesterday). She has praised my writing and I know my help saved her skin so I think I am within rights to ask for such a substantial increase. I just don’t know how to ask for it.

Help!!!!!!