@Mary64 That is one incredibly stupid host! Especially if she doesn't want to negotiate with you. Here's the deal, if you have a confirmed reservation and have already paid for it, you don't have to do anything. The host may cancel the reservation, but if she does, she will not be able to re-book the dates you had reserved. So instead of getting your payment, she'll get zero.
Airbnb has severe penalties for hosts who cancel confirmed reservations. The dates of the reservation are blocked and cannot be re-booked, there is an automatic review on her listing saying she cancelled your reservation (a kiss of death for guests looking to make a reservation with her) and there is a financial penalty. She's probably just figuring that out now.
I would contact Airbnb and let them know that this host is asking for more money so they can make a note of it(have the reservation code handy since they'll ask for it). This is especially importants since she had Instant Book turned on in her listing. After May 19 when the new Terms of Service go into effect, it will be a violation to dishonor a confirm reservation because the host wants more money. I assume this could lead to the host's listing being removed from Airbnb.
Other than contacting Airbnb, there's nothing for you to do. Hope this helps!