Musculoskeletal Strain or Thoracic Outlet Syndrome: Given the description of pain under the shoulder blade radiating into the shoulder, arm, and elbow, musculoskeletal strain or thoracic outlet syndrome is a plausible diagnosis. The pain's persistence even with a change in sleeping position suggests a possible issue with muscle or nerve compression that is not solely dependent on sleeping posture.

Costochondritis or Tietze's Syndrome: Inflammation of the cartilage that connects the ribs to the breastbone can cause sharp pains in the chest that may radiate to the back and arms, potentially exacerbated by certain positions or movements.
Gallbladder Disease: Although less directly related to the symptoms described, gallbladder issues can cause pain in the right upper back that sometimes radiates to the shoulder and arm. However, the left-sided nature of the pain makes this less likely.
Peptic Ulcer Disease: Given the patient's history of celiac disease, which increases the risk of other gastrointestinal issues, peptic ulcer disease could be considered, especially if the pain is related to eating or has a burning quality.

Pulmonary Embolism: Although the description does not classically suggest pulmonary embolism, any sudden onset of back or chest pain, especially if accompanied by shortness of breath, cough, or recent history of immobility, requires consideration of this potentially life-threatening condition.
Aortic Dissection: Severe, tearing chest pain that radiates to the back is a classic symptom of aortic dissection, a medical emergency.
Spinal Cord Compression or Cauda Equina Syndrome: These conditions can cause back pain radiating into the limbs and are emergencies that require immediate medical attention to prevent permanent neurological damage.

Pancreatitis: While more commonly causing abdominal pain, pancreatitis can sometimes cause back pain, especially if the pancreatic inflammation affects nearby structures.
Spondyloarthritis or Ankylosing Spondylitis: These inflammatory conditions can cause back pain and stiffness, and while less likely given the radiating nature of the pain, they should be considered in patients with a history of autoimmune diseases like celiac disease.
Neoplastic Processes: Tumors affecting the spine, ribs, or surrounding tissues can cause persistent back pain that radiates to the arms or shoulders. Given the patient's known celiac disease, the potential for other autoimmune or inflammatory conditions, or even a paraneoplastic syndrome, makes considering neoplastic processes important, albeit rare.