Slept on shoulder funny , pain persisting for 4 weeks now , x-ray came back normal..what's going on? still have full range of motion

Muscle strain: The cause of a soft tissue injury or pain coming from an injured nerve will often not show up on a plain xray. Multiple syndromes of soft tissue injury are possible, especially if there is a history of rotator cuff injury in past. Nerve impingement syndromes also may explain this type of pain. It may be possible to narrow down the cause with a detailed physical exam. See your doctor if it persists.