here's a website for cause and cure for your pains..
Back in the day when I had severe back problems (muscle spazim) the Doc told me it was primarily from out of shape stomach.. I recieve an exercise chart to strenghten the problem area.. The following link may give you some tips to straighten you out...
http://www.drbookspan.com/NeckPainArticle.htm
You might also make sure you are staying hydrated..
A worse solution take a couple across the counter pain relievers 30 to 40 minutes before show time, also a back brace. There are diferent types for upper or lower back pain.
Good luck getting your back and shoudler painless.
Gordon