Hi,Your pains are confined to your upper arm and shoulder which makes it a local problem most likely.when you raise the arm is the pain in the shoulder or in the front of the arm? The possibilities are a rotator cuff tear or a biceps tendon inflammation or tear which explains the weakness . Lying on one side may cause some temporary stiffness in the shoulder but not in the way you have described. Primary muscular problems though can cause weakness is uncommon and so less likely. An X ray is needed and will show any bony problems(like arthritis) and an MRI will reveal the status of the rotator cuff and biceps tendon and other changes. Try warm oil massages and hot packs, physical therapy by ultrasound massage will be beneficial along with Diclofenac K or Celecoxibs