It is very comforting to know that I am not the only one suffering from this. I first experienced this a few years back after editing a feature many months. It is almost exactly the same type of pain in the same part of the shoulder. It got so bad that I went to a chiropractor for a couple of months and it got the pain to a manageable level. It has recently returned and I am mostly just treating it with Ibuprofen (800mg) but have noticed just how poor my posture is.
I am now entering a new full time gig as Senior Editor and I am going into this job with ergonomics in mind. I am considering a stand up work station and the height of the monitors.
I wish the best in taking care of yourself while doing the editing grind.