Why do I have a bump on the back of my neck?

I am 17 and I've had this for years now. But in recent years it's been way more noticeable. it's a big bump that's a whole hand under the hairline. It appears to be bone. But It hurts after a while if I don't stand or sit straight. It also makes my neck a bit stiff when looking down. It's one of those bumps that appear over night or are small, I've had it for years and it just gets more noticeable with time. Does anyone have an opinion of what this is?