What is this lump on the back of my neck?

Under the skin, at the top of my back/very bottom of my neck I have a lump. It's very hard, it feels like bone. Its slightly sore and I don't understand what it is. It's so bony that I am tempted to think its a vertebrae but unless you're very thin you shouldn't really be able to feel your vertebrae without bending over far. I does not move when I swallow. I'm a little worried, things like this that might affect the spinal cord can be serious. Looking for experienced opinions on whether I have need to worry or not.