My neck has been fine since I woke up, then as I was sat down in a slouch, I moved to sit up properly, and now the top of my neck is causing sharp pain all down my back and at the base of my skull and in my neck when I put my head in a too-upright position. It also hurts if I put pressure on the top of my head, or turn it, but I seem to be able to get more flexibility if I turn my head as far as I can, then back to normal, then back again. What is this and how can I treat it?