Gastroesophageal reflux disease (GERD): This condition is a common cause of chest pain that can occur while sitting, especially after eating. The pain is often described as a burning sensation and can be exacerbated by lying down or bending over.

Musculoskeletal pain: Strained muscles or costochondritis (inflammation of the cartilage that connects the ribs to the breastbone) can cause chest pain that worsens with movement or deep breathing, and may be noticeable while sitting.
Anxiety or panic disorder: These conditions can cause chest pain or discomfort, often accompanied by other symptoms such as shortness of breath, palpitations, or fear of losing control.
Pericarditis: Inflammation of the sac surrounding the heart can cause sharp chest pain that may improve with sitting up and leaning forward.

Myocardial infarction (heart attack): Although less likely in certain populations (e.g., young individuals without risk factors), chest pain while sitting could still be a sign of a heart attack, especially if accompanied by other symptoms like arm or jaw pain, shortness of breath, or sweating.
Pulmonary embolism: A blood clot in the lungs can cause sudden, severe chest pain that worsens with deep breathing or coughing, and may be fatal if not promptly treated.
Aortic dissection: A tear in the aorta's inner layer can cause severe, tearing chest pain that may radiate to the back, and is a medical emergency.

Pneumothorax: A collapsed lung can cause sudden, severe chest pain that worsens with deep breathing or coughing, and may be accompanied by shortness of breath.
Esophageal rupture or perforation: A tear in the esophagus can cause severe chest pain, often accompanied by difficulty swallowing, vomiting, or fever.
Costochondritis due to underlying conditions: Certain conditions like rheumatoid arthritis, lupus, or sarcoidosis can cause costochondritis, leading to chest pain while sitting.