There are many potential causes of left arm pain, so it is difficult to give a definitive answer without knowing more about your specific situation. In general, however, arm pain is most commonly caused by overuse or injury to the muscles, tendons, or joints in the arm. It can also be caused by compression of the nerves in the arm, such as carpal tunnel syndrome. Additionally, arm pain can be a symptom of more serious medical conditions, such as heart attack, stroke, or cancer. Therefore, if you are experiencing arm pain, it is important to see a doctor to determine the cause.