Hi Mike,
Long thoracic nerve originates in the brachial plexus...therefore, you need an assessment from someone who can see if you neck and thorax are working properly. Often it could be a joint problem, overactive muscles or a muscle imbalance etc.
The pain may be from the Long Thoracic Nerve but has anyone answered the question of why??
Also, could you be more specific as to where your pain is - perhaps deownload a bodychart from google images and use MS Paint to colour in where your pain is...perhaps those who said it is the long thor n have got it wrong!
Cheers!