why did pain that lasted so long in my shoulder move to the left shoulder

There could be a few reasons why the pain in your shoulder moved to your left shoulder. One possibility is that you were overcompensating for the pain in your right shoulder, causing strain and discomfort in your left shoulder. Another possibility is that the initial injury or issue that was causing pain in your right shoulder has now also affected your left shoulder. It is important to consult with a healthcare professional to determine the underlying cause of the pain and receive appropriate treatment.