bluescouse, have you looked at the ergonomics of you computer setup?
Some years ago I had severe pain in my right shoulder if I raised my arm above horizontal. My doctor said rotator cuff and told me to take 3 Ibuprofin 4 times a day (upset my stomach). Then my left shoulder started to hurt. Since I was sure that rotator cuff pain is not contagious I started to look for another reason.
It turned out that my keyboard an mouse were too high. Lowering them 4 inchesgave me almost instant relief, and now my shoulders are pain free.
Google the phrase "PC ergonomics" and you should find a ton of sites that deal the correct setup.
Good luck
Nhlett