I think what emily s discussed, frozen shoulder, sums up the pain I now have in my left shoulder. I am usually very active, usually run and/or walk for at least 6 miles a day and I am in very good physical shape. I do the stretching required before and after my exercise.
However, recently I have had a lot of typing to do and my left shoulder started hurting about 1.5 months ago. I think my body just does not like inactivity. I had the same thing happen to my right shoulder about 2 years ago when I had to sit a lot and type. The pain in both shoulders was the same and very intense. Thorough exercise, my right shoulder healed quickly and now my left shoulder is getting better, although it is taking longer. I still cannot get my billfold out of my left rear pocket and I have difficulty getting a shirt on and off.
The moral: If you are very active do not stop, especially if you are past 55 years old.
Good luck. Getting older is not all it is cracked up to be.