How strange! What is her feedback like? I can't imagine a sane person doing this unless they really thought they had bought a fake bag. There is really nothing she can do except threaten you with e-mails...but how awful!
Or she could have learned about this "trick" later on, and now acting on it to see if she could get away with it. If I were the buyer in this situation, I wouldn't blindly email the seller saying it is fake. I would provide proof of its non-authenticity through either JC or some sort of information as to why I think it is not authentic.