I was having a lot of mid back pain, also some pain and stiffness in my right arm. I found out by accident what was causing it. I was just reviewing a Youtube vid on Traditional Chinese Medicine and tongue analysis. Looking at my tongue I saw it had a white coat. It didn't go away with brushing. CTM says a white coated tongue can be related to liver problems, so I did a liver flush. The tongue improved immediately, and so did the aches and pains.
I saw a site by a masseuse who has noticed that back pain, arm pain and stiffness, shoulder pain and even left hip pain can be caused by gallstones.
And btw I found Stone Free tablets to work great and to be a lot less hassle than liver flushing. They also don't make you feel tired after using them. Stone Free has two problems, though. The tabs are slightly acidic. You have to be sure to compensate for that with additional alkalinity in your diet. Also, unlike with liver flushes, you don't have any outward visible signs of the stones going away (it also treats kidney stones btw).