Obviously, she is trying to take you. Should've, could've, would've had a contract or no-I bet she would STILL be grumbling about something. Even though it wasn't in writing, she new the deal up front (whether or not the she knew of the cost to purchase more pictures) and she agreed to that.
I know it is hard, and you have to go with your gut, but I would REALLY have a hard time giving her more. Sorry you're going through this.