Rotator Cuff Tendonitis
This can be caused by over use or a one time trauma. Pain is generally on the outside of the shoulder and can go down the outside or front of the arm above the elbow. Pain is usually worse with reaching overhead or reaching back like tucking in a shirt. It is a condition that affects the muscles that control the motion of raising your arm. It can be a very painful and debilitation condition. If not properly treated, it can lead to a rotator cuff tear, where surgery is usually recommended.
It is best to consult a medical professional to determine extent of injury. In the meantime, it is best to rest and use ice and limit painful activities, especially overhead activities.