You have my sympathy - hope my explanation might be of some use...
I'd been suffering similarly for a few months, thinking I'd wrenched my shoulder moving my old computer monitor. Sudden movement, most stretching, reaching behind my back, and sleeping were painful. My GP tried painkillers and anti-inflammatory pills, but they just made me light headed and dizzy. He then suggested I visit the physiotherapist - there it was diagnosed as 'frozen shoulder' and I was given some exercises which helped a little, but I was still suffering.
Then I went to a local chiropractor, and after the first session I felt better - he referred me to the sports massage therapist, and those sessions helped too. After 2 months of these treatment sessions most of my shoulder/arm motion has returned - not back to normal yet, but they advised me on some different daily exercises and things are still 'on the mend'.
I do a lot of computer work in the office and at home, add to that lots of guitar playing (we always try to stand rather than sit when performing - better for vocal projection etc), so it will no doubt be something I need to keep in mind and not forget the exercises!
(jiva - Jimmy & Val)