I had an inflamed rotator cuff (left shoulder - right handed in everything, including golf). At the time I had no idea what was wrong but I couldn't lift my arm higher than parallel to the ground without significant pain. Sleeping was difficult because gravity no longer pulled my left arm down.
An orthopedic physician diagnosed the problem. Gave me some prescription anti-inflammatory pills which I took for a month. I spent four weeks rehabilitating the shoulder. Started with light stretching, then moved on to light weights and slowly progressed. Missed 26 days of golf in the month of August (oh the agony!!)
The key is to get the inflammation down as soon as possible. The longer one allows the rotator cuff to be irritated, the worse the inflammation and the longer the rehab time.
Even today, if I sleep with my arm/shoulder flexed up near my head, the shoulder will start to bother me. My day to day situation is fine otherwise. No problem with day to day activity, playing golf, tennis, lifting weights, etc ... My left arm does seem to be permanently weaker but not too bad.