Why Is My Arm Hurting From Past Few Months?

Yes, my left arm has been hurting for a few months now. Stated in shoulder and hurt when I slept on it. Depending on how I move my arm I feel horrible pain either on the front of my arm or behind my arm. Feels like a rubber band and hurts extremely. I had a a cortisone shot and it did not work. I cannot reach up or pick up anything without pain. What can this be