I had the same symptoms (couldn't lift my outstretched arm to be parallel with the ground, at 90 degrees to my body). It wasn't related to bass playing. Physio determined it was due to tension in my neck muscles, and several sessions of neck massage later it was fixed, with no re-currence. The Osteo/Physio route is good, but worth a visit to the GP first - in my case the GP gave me one of the practice 'credits' so it was done on the NHS.