I've dealt with it in my right shoulder on and off for like eight years. Usually it's aggravated by a repetitive task at a job or something. I've been to the doc for it, several times. Had an x-ray done. I'm always told it's tendinitis. Well for the past two weeks when I sleep on my right side, I wake up in excruciating pain. It feels like there's broken glass in my shoulder. After I get up though and move my arm around it feels better. Now I've been a side sleeper all my life. My shoulder has never hurt just from sleeping on it. I have no clue what could be causing this.