Hard, painful lump on back of head?

Hi, I'm a girl and almost 15. A few days ago I noticed a hard, painful lump on the back of my head. It's at the bottom of my skull I think and at the top of my neck on the left side. It's maybe about an inch away from the middle of my left ear. It feels about the size of a quarter, maybe a little bigger. My mom looked at it and said it's a cyst but I read that cysts are softer than this. It hurts when I touch it and when I sleep on it and it doesn't move at all. I'm getting kind of nervous about it after reading more on the differences between cysts and cancerous lumps. Should I see a doctor? Thanks :)