If you look in this picture, you can see her spine actually bends at a point where the hump is. The second picture I just traced her spine so it's easier to see.
How long have you had her? Has she always had this? What color is her poop? Does she poop daily?
I've had her for about a year and half, i got her from a friend who couldn't take care of her anymore. She's always had the hump. Her waste is tan, and I'm guessing she goes everyday... not that i really watch for that.
If she's had it that long and is otherwise healthy I wouldn't worry about it. May be birth defect or injury that healed funky or a tumor which isn't causing any real issue.