Help! I don’t know what’s wrong with my rat ):

Hi! I’ve had this girl for a little over a year now, she’s 2 and a half years old. I noticed about a week ago this small patch of skin showing on her but it grew and now it’s pretty big and I don’t know what happened or if she’s scratching it off?? I don’t know what’s going on with her. Her sister is fine and doesn’t have any of these hairless patches on her so I’m just confused and not sure what to do ):