What Causes Pain In Shoulders And Arms?

Hi, Im relativlely young and keep active by playing sports at least once a week. Since the last few months iv been getting pain in me outer right shoulder/upper right arm which usually increases by leaving the arm straight down. If im sitting with the arm bent from the elbow i can go through days without feeling pain but if i ever leave my arm straight then i start feeling pain in my right shoulder within 10 mins and worsens the longer i leave it. The pain i feel is usually related to how long i leave my arm straight or if i do some lifting. What could be the cause of this pain?