Since the seller initially said she had 10 yards available, then made the adjustment after the sale, I have to think she sold the rest of it to someone else and was trying to finagle to be able to sell her entire 10 yards... But not checking with you first was definitely NOT COOL.
You should NEVER leave bad feedback before trying to work things out with the seller. Most would rather give your money back and let you keep the goods than get a negative feedback rating to hurt future business. Contact her and tell her your feelings and then decide on feedback, based on how satisfied you are with her response. Always give them a chance to make it right, first. Negative feedback is a serious business, as many people are making their income selling on Ebay.