Musculoskeletal Strain: The onset of pain after bending over and lifting a chair suggests a mechanical cause, likely a strain of the intercostal muscles or a costovertebral joint injury, which is common with heavy lifting or bending.

Costochondritis: Inflammation of the cartilage that connects the ribs to the sternum, which can cause pain in the chest wall, including the posterior aspect of the ribs, and can be exacerbated by physical activity.
Rib Fracture: Although less likely without significant trauma, a stress fracture could occur, especially if the patient has osteoporosis or other bone density issues, and the mechanism of injury (lifting and bending) could potentially cause a fracture.
Intercostal Muscle Strain: Similar to musculoskeletal strain but more specifically involving the muscles between the ribs, which can be injured during heavy lifting or bending.

Pulmonary Embolism: Although the history does not strongly suggest this, any chest pain, including rib tenderness, warrants consideration of pulmonary embolism due to its high mortality rate if missed.
Pneumothorax: Trauma to the chest, even if minor, can cause air to leak into the space between the lung and chest wall, leading to a pneumothorax, which is a medical emergency.
Myocardial Infarction: Chest pain, even if localized to the ribs, can be a presentation of myocardial infarction, especially in women or individuals with atypical presentations.

Tietze's Syndrome: A rare inflammatory condition of the cartilages and surrounding tissues of the ribs, which can cause severe pain, usually in the anterior chest wall but potentially affecting the posterior aspect as well.
Sickle Cell Crisis: In patients with sickle cell disease, a crisis can cause bone pain, including rib pain, due to vaso-occlusion.
Metastatic Disease: In patients with a history of cancer, metastases to the ribs can cause localized pain and tenderness.