Interesting, a trapped nerve is usually a get out clause when people present symptoms the doctor does not understand sufficiently to make an accurate diagnosis. Another is some kind of viral infection. The problem in all fairness is that a GP's work load in the UK prevents them spending long enough with a patient to make a clear diagnosis.
A nerve problem could as you say cause pain and loss of movement in the arms and shoulders and could also cause muscles to atrophy, leading to a miss-directed diagnosis.
A brief search reveals that symptoms are exacerbated by bed rest. Now there's a surprise.
www.medicalnewstoday.com/articles/147083.php
There are many theories as to the cause of fibromyalgia and
What is fibromyalgia? What causes fibromyalgia?
The word fibromyalgia comes from the Greek myos meaning "muscle", Greek algos meaning "pain", and New Latin fibro meaning "fibrous tissue".
Fibromyalgia is a common and chronic disorder. When a health illness or condition is chronic it means it is long-lasting.
Even though fibromyalgia is frequently referred to as an arthritis-related condition, it does not cause joint damage or inflammation, as arthritis does. Neither does fibromyalgia cause damage to muscle and other tissues. However, it is similar to arthritis because it causes severe pain and tiredness, and can undermine the patient's ability to go about his daily activities. Fibromyalgia is seen as a rheumatic condition. A rheumatic condition is one that causes joint and soft tissue pain. What are the signs and symptoms of fibromyalgia?
Morning stiffness
Headaches
Irregular sleep patterns
IBS (irritable bowel syndrome)
Painful menstrual periods (Dysmenorrhea)
Hands and feet tingle and can feel numb
Restless leg syndrome
Sensitivity to cold or heat
Fibro fog (memory problems, cognitive problems).
The following signs and symptoms are also possible
Widespread pain
Problems with vision
Nausea
Pelvic and urinary problems
Weight gain
Dizziness
Cold/flu like symptoms
Jaw pain and stiffness
Skin problems
Chest symptoms
Depression
Anxiety
Myofascial pain syndrome (pain/tiredness in muscles and adjacent fibrous tissues)
Breathing problems.