I am not a doctor, but can tell you my experiences.
First if you are getting pain see a Doctor, and a maybe message therapist. Stop shooting, and don't try to work through the pain. It needs to heal before any other stimulation is given to it. Only a doctor should let you know when to exercise, etc.
Causes could be a number of things including age, over exertion, over rotation in bow arm.
Which side is hurting you bow or draw? Draw sides are usually impingements. Bow sides are usually stains/tears. Bad news is that if it a bad tear, it usually needs surgery.