It is likely to be a sign of angina/heart attack. Angina is the pain that is felt as a result of lack of oxygen-rich blood being delivered to the heart. Along with the symptoms felt in the chest, angina has the potential to refer to the shoulders, arms, neck, jaw, or back. The most common symptom of angina radiates to the left shoulder and down the inside of the arm.
Obviously, shoulder pain could also be caused by other non-cardiac conditions. The determination of whether shoulder pain is caused by the heart could only be determined after you get examined.