My first thought was the same as KMG- that it's a thermal burn from being under an excessively-hot heat source. For safety, all heat sources should be controlled by a thermostat & checked on regularly- burns are not only painful for snakes, but they can be life-threatening or subject a snake to an infection that's also life-threatening.
IF this isn't a burn, it's also possible your snake has been wedging herself under something in her cage, & then scraped her way free of whatever it was. Could even be from the top of the enclosure, if she has things to climb on that allow her to scrape her body on the ceiling.
How long has she had this? Any blisters?
Reply With Quote