Not being paid for graphic design work. Client keeps trying to reduce the price after they have the work?

So I worked on a campaign and made many designs for a Judge. She knew I was charging her and I got paid one time. So the second round came up and I did a ton of art and then she decided to give me two more huge projects last minute. So I told her her second invoice would be coming. She was mad because I was told to make a mailer for her, worked on it for hours, only to have her send a drawing of what she wanted. She doesn't understand that I still get paid for my time? I started remaking it anyways saying that I wouldn't charge extra for the 2 projects (as in the price I billed her for was for everything else) she then was unhappy with the fact that I couldn't do two mailers overnight and had someone else do the mailers. However she had them copy off of my work and replicate the first mailer. Now she wants me to reduce my price. I'm telling her that I was being nice and doing the last 2 mailers with out charging more. She won't listen. What can I do?