As an Osteopath I can tell you that "fibrositis" is medical language for "inflammation of the fibres". Considering that there are fibres present in muscle, tendon, ligament, fascia and all connective tissue it is, therefore, nothing more than a doctor's clever way of saying "I don't know what's wrong with you, but you've got pain".
I get just as annoyed when I hear medical doctors tell a patient "You've got sciatica" when they have pains down the back of the legs. Sciatica ia a symptom, not a diagnosis.
Pain in the shoulder can be have many causes. Age, occupation, hobbies and sporting interests of the patient must be taken into account, together with any history of trauma, either recent or historical. The way in which the body works, with one part of the body having an effect on another, can cause remote pain. For instance, shoulder pain can be caused by arthritis of the hip, where the opposite shoulder is used for balance when walking.
Shoulder pain can also be indicative of internal organic problems.
Suggest you see an Osteopath or Chiropractor near you for a more accurate diagnosis than you are likely to get from a GP.
Colyn.