I would just tell her that your paydays are your paydays and no she can't pay you on a different day. Maybe she needs to budget differently - not your problem.
Tell her face to face. Don't back down. If we don't enforce our own policies then who will!
Also, she told me, "You said at the beginning that this would be fine", when in actuality what I told her is that we would see how it goes and we would try to work it out. Well, now that I'm licensed I want to be 100% professional, and that means no one gets special treatment. I also want to be 100% organized with my bookkeeping!