I have this hump on the bottom of my neck?

I noticed a while back that I have a little hump on the back of my neck. Its towards the bottom and it looks as if I have a hunchback. I have never had good posture, so could that be it? Bad posture? My sisters have it also, mine is actually the least bad out of all of them. We are all overweight, and my little sister is obese, she is 12.. my older sister is 22 and I am 14. My mom has major thyroid problems and I have heard that could aslo be a contributer.