I have a lump on the back of my neck in the middle next to the top of my spine.It appeared about 6 months ago the size of a pea but has grown now to about the size of a marble,its very hard and has started to hurt a little as if I have been sleeping the wrong way.When you touch it it moves around slightly.Has anyone got any advice on what it could be as Im starting to get a little worried especially as it has grown.