Strange thing?

There's this girl in my school, really nice, a good friend, and she seems normal and healthy. But she has this strange thing going on with her . The center of her lower back, just above her , sticks out suddenly. Her doesn't even seem to be there. Its kinda like a big boney hump in her lower back. Its very strange and it makes her look like she's waddling from side to side. Sometimes i think its a tail or something but no one really knows what the hell it is!!! Any ideas?