I don't think you did anything wrong and certainly there isn't anything wrong with you. I think she feels some shame for her behavior and wanted to be honest with you, a good intention maybe, but after having a romantic evening was probably not the best time to disclose. I imagine you showed some surprise and dismay on your face. We all want to feel we are special to our partners. If it had been me in her place I would have kept that information to myself. My husband doesn't know of all my early experiences 28 years down the road. It would have only hurt him I think.