She hasn't hatched any chicks recently has she? The only time I've ever seen a hump like that, it was when a baby chick crawled up there in momma's feathers. 😊
I assume you can't catch her to investigate it, but when she goes to the coop tonight, I'd take her in the house and thoroughly check her over. This could be everything from an infection to a boil to who knows.
When you've checked on her and found out, please let us know!