Does she roost normally? Could be from resting/rubbing on a perch, or laying in bedding rather than roosting (especially if the bedding is wet or very dirty). I'd clean it off with some hibiclens or diluted betadine, apply some plain triple antibiotic ointment or plain neosporin and see if it improves. If she's resting on a roost you may need to pad it to prevent this. Sometimes they will do this if they are having leg or foot issues, or if they are a heavy bird. If she's laying in bedding rather than roosting I would try to figure out why and get her to roost. If that's what's going on then if left alone it can possibly become a breast blister, which is like bumblefoot but on the breast/keel.