RSD... does the arm swell and have skin color changes {red, blue, mottled}
hypersensitive to touch, even breezes or clothing ?

I was thinking it might be thoracic outlet syndrome with your heavy work and adding in computer uses.

both our RSD & TOS forums have a good amount of stickys to explore
Maybe shappy already mentioned those.

A couple of our TOS members did have frozen shoulder over the years..
I think the constant upper body spasms play a part there.

hope you can get a handle on it soon, or at least the pain reduced for now...