Rib Fracture: This is the most likely diagnosis given the history of a fall and ongoing left-sided rib pain. Even though the chest X-ray is non-specific, rib fractures are common after blunt chest trauma and can cause persistent pain.

Costal Cartilage Injury: Similar to rib fractures, costal cartilage injuries are common after blunt chest trauma and can cause persistent pain. The non-specific chest X-ray does not rule out this possibility.
Musculoskeletal Strain: The fall could have resulted in a musculoskeletal strain of the chest wall muscles, leading to ongoing pain.
Soft Tissue Injury: Soft tissue injuries, such as bruising or contusions, can also cause persistent pain after a fall.

Pulmonary Embolism: Although less likely, a pulmonary embolism could present with chest pain and must be considered, especially if there are other risk factors such as immobilization or deep vein thrombosis.
Pneumothorax: A pneumothorax could be present even if the initial chest X-ray is non-specific, especially if it is a small pneumothorax.
Myocardial Infarction: Chest pain after a fall could also be due to a myocardial infarction, especially in older adults or those with cardiovascular risk factors.

Osteochondroma or Other Bone Tumors: Although rare, bone tumors such as osteochondromas could cause rib pain and should be considered if other diagnoses are ruled out.
Sickle Cell Crisis: In patients with sickle cell disease, a crisis could cause chest pain and should be considered in the appropriate clinical context.
Thoracic Aortic Dissection: This is a rare but life-threatening condition that could present with chest pain after a fall, especially in older adults or those with hypertension.