Muscle Strain (6th rib): The patient's presentation of left sided chest pain and a history of musculoskeletal injury makes muscle strain, particularly of the intercostal muscles associated with the 6th rib, the most likely diagnosis. This type of injury is common and can result from direct trauma, overuse, or sudden contraction of the muscles.

Muscle Tear (6th rib): Similar to muscle strain, a tear in the intercostal muscles or other muscles in the chest wall could cause significant pain and is a plausible diagnosis given the mechanism of injury.
Costochondritis: Inflammation of the cartilage that connects the ribs to the sternum, known as costochondritis, can cause chest pain that may be exacerbated by movement or deep breathing.
Rib Fracture: Although less common than muscle strain, a fracture of the 6th rib could also cause left sided chest pain, especially if there was significant trauma.

Myocardial Infarction (MI): Although less likely given the musculoskeletal context, myocardial infarction (heart attack) can present with chest pain and must be considered, especially in patients with risk factors for cardiovascular disease.
Pulmonary Embolism (PE): Sudden onset chest pain, especially if accompanied by shortness of breath, could indicate a pulmonary embolism, which is a life-threatening condition.
Pneumothorax: Air in the pleural space can cause chest pain and shortness of breath. This condition requires immediate medical attention.

Tietze's Syndrome: A rare inflammatory condition affecting the cartilages and surrounding tissues of the ribs, which can cause chest pain.
Sickle Cell Crisis: In patients with sickle cell disease, a crisis can cause chest pain due to vaso-occlusive episodes affecting the lungs and chest wall.
Osteomyelitis or Osteitis of the Rib: Infection of the bone or bone marrow of the rib, which is rare but can cause localized pain and other systemic symptoms.