She doesn't actually "suddenly" call herself that when she's reunited with her father. In the early part of the book, during a chapter that is detailing the family history, it says that when Gerald and Ellen O'Hara's first child is born, they named her Katie Scarlett after Gerald's mother. It further states that Gerald was disappointed, because he had wanted a son, but he was nonetheless pleased enough with his black-haired baby daughter to get rip-roaring drunk. See what an aficionado I am?