Pain in backside of left shoulder.....when sleeping using the left side....gets pain and numbness on the shoulder.

Suspect tendonitis: Could be a variety of otehr cuases but this would be my first suspicion,. Discuss with your physicain.

Examination required: Many possibilities - pinched nerve (radiculopathy) from neck vs shoulder issue vs referred pain/numbness from chest or other internal structures. A doctor review wil help sort out likely possibilities here, arrange tests if not clear and then help with treatment. All the best.