Gastroesophageal Reflux Disease (GERD): This condition is a common cause of intermittent chest and back pain, especially if the pain is related to eating or lying down. The pain can radiate to the back and be felt under the shoulder blade or breast area.

Musculoskeletal Strain: Strain or injury to the muscles or ligaments in the back or chest can cause intermittent pain in these areas. Activities that involve heavy lifting, bending, or repetitive motion can contribute to this condition.
Costochondritis: 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 be felt under the shoulder blade or breast area.
Gallbladder Disease: Gallstones or inflammation of the gallbladder can cause pain that is typically felt in the right upper abdomen but can radiate to the back and be felt under the right shoulder blade.

Myocardial Infarction (Heart Attack): Although less common in women and younger individuals, a heart attack can present with atypical symptoms including pain in the back or arm. It is crucial to consider this diagnosis, especially if accompanied by other symptoms like shortness of breath or arm pain.
Pulmonary Embolism: A blood clot in the lungs can cause sudden, severe pain that can be felt in the chest and back. This is a life-threatening condition that requires immediate medical attention.
Esophageal Rupture or Perforation: A tear in the esophagus can cause severe chest pain that radiates to the back, along with difficulty swallowing or vomiting.

Pancreatitis: Inflammation of the pancreas can cause severe abdominal pain that radiates to the back. Although the pain is typically more central, it can be felt under the shoulder blade in some cases.
Pneumothorax: A collapsed lung can cause sudden, severe chest pain that can radiate to the back and shoulder area.
Thoracic Aortic Dissection: A tear in the aorta can cause severe, tearing chest pain that radiates to the back. This is a rare but life-threatening condition.