Sounds like you've either trapped a nerve or put a muscle in spasm - happens to me ALL the time posy so I know how it feels! I actually see a chiropractor regularly though to get mine sorted, but you can either get someone to massage it for you (find the spot that really hurts and get them to use their thumb and gently but firmly start rubbing it in a circular motion and gradually press harder) - it hurts like hell at first but will feel better once it loosens up. Or put frozen peas or ice pack on it for 15 mins every hour and take ibuprofen to take down the inflammation or have a hot bath and really try to relax. I would go and get it looked at though if it gets any worse or doesn't go away in a few days cos it can get worse!