Musculoskeletal strain: This is the most likely diagnosis given the context of playing golf, which involves twisting and bending, and the sharp pain experienced when bending over. The musculoskeletal strain could affect the intercostal muscles or the abdominal muscles, leading to pain in the subcostal region.

Costochondritis: Inflammation of the cartilage that connects the ribs to the sternum, which can cause sharp pains in the chest or subcostal area, especially with movement or deep breathing.
Intercostal muscle strain: Similar to musculoskeletal strain but more specific to the muscles between the ribs, which could be irritated from the golfing activity.
Gastrointestinal issues (e.g., gastritis, ulcer): Although less directly related to the physical activity of golf, bending over could exacerbate gastrointestinal discomfort or pain, especially if the individual has a pre-existing condition.

Pulmonary embolism: Although unlikely given the context, any acute onset of sharp chest or subcostal pain, especially if accompanied by shortness of breath, cough, or other systemic symptoms, necessitates consideration of a pulmonary embolism due to its high mortality rate if missed.
Myocardial infarction: Chest pain, even if localized to the subcostal area, should prompt consideration of cardiac causes, especially in individuals with risk factors for coronary artery disease.
Splenic rupture or infarct: Given the location of the pain (left subcostal), a splenic issue could be considered, especially if there was significant trauma during golfing or if the patient has a history of splenomegaly.

Pneumothorax: A collapsed lung could cause sharp chest pain, but it would typically be accompanied by shortness of breath and might not be directly related to bending over after golfing unless there was an underlying lung condition.
Diaphragmatic rupture: This is a rare condition that could result from severe trauma but might be considered if the patient experienced significant force to the abdomen or chest during golfing.
Pancreatitis: Inflammation of the pancreas can cause severe abdominal pain that radiates to the back, but it's less commonly associated with sharp subcostal pain and bending over unless the pancreatitis is severe and involves the tail of the pancreas, which is located near the spleen.