Shoulder pain when I lift my arms up to the side. What could this be?

Hard to say. There are multiple potential causes. The two most common would be a rotator cuff problem or shoulder instability. The rotator cuff muscles raise your arm away from your body and when they are injured raising your arm can be painful. Some people will have an unstable shoulder that can shift out of place if the arm is brought away from the body. A good exam can tell the cause. Thank you.