I’m trying to find a book that I read in the late 70’s. It was a paperback book, probably a teen novel about a girl whose name was Katherine (I think). She goes into an antique store or an attic and sees a picture of herself which turns out to be a picture of her great great great Grandmother. She has a dream and goes back in time and lives her grandmother’s life in great detail, for years. It seems to be a life in the Middle Ages.
She suddenly wakes up from her dream and is told that she has only been asleep for about 15 minutes and that in fact, she had a seizure. During this very short seizure, she lives her ancestor's life.
I believe the cover of the book had a portrait of the girl from the Middle Ages in it.
Any help would be greatly appreciated. I loved this book and would be so happy to find it.