Mult possibilities. There are multiple reasons why your shoulder could be hurting. Rotator cuff pathology, impingement syndrome, bursitis, biceps tendinitis, or referred pain from your cervical spine are among the most common possible reasons. The best way to find out what the cause of your pain is is to have a thorough physical exam with imaging from your doctor.
See your doctor. This does not look like a problem that can be treated on the web.