I think it might be time to contact your doctor and make sure that you don't have some kind of impingement going on. This back and forth type thing sounds like what I suffer from. Since you complain about it happening every now and then, I would suspect some type of imflammation is going on. You could also have a small rotator cuff tear in that shoulder. My orthopedist says he sees this often in worman that are large chested.
The expert on this kind of thing is an orthopedist. I get steriod injections in my shoulder 3 times a year, to try and help mine, although I am now at a stage where surgery is my only option. Let's hope the doctor's can get this settled down for you...
I know your hurting...I've been there...more times that I can count.
Heather
No, I've never injured it that I can recall, and I can't find any correlation between activity preceding the onset and the timing. I can't lie down at all without pain, not on my back, sides, or stomach. It's on the left, so it's not the arm I've ever used much in sports or anything (I'm right handed). It just seizes up every few weeks for no apparent reason, the pain lasts for about three days, and then it goes away.
E
I have an old shoulder injury that damaged my rotator cuff. I had surgery to repair it and now have developed a large area of calcium deposits that cause shoulder pain to flair up from over-use. It expecially hurts to lie on my back, when it's acting up. Is it possible that you overused your shoulder to have caused this sudden problem? Do you remember hurting yourself?
Heather