Originally Posted by Chas Tennis
You have an undiagnosed shoulder injury. Tennis is stressful for the normal shoulder and much more stressful for an injured shoulder. There is a reasonable chance that it will not spontaneously heal but will progressively get worse.
If the injury involves tendons they can heal defectively if stressed while healing (even over a very short time). Research tendon healing times, TW and elsewhere. http://tt.tennis-warehouse.com/showthread.php?t=442912
Location of the pain could be impingement, a common injury that often/usually gets worse with the same activity. The supraspinatus muscle-tendon is on the top and the most common location of shoulder impingement injury. I had mild tears of my supraspinatus tendon of my non-tennis arm probably from gym work. With use impingement tends to cause swelling and then the impingement gets worse. Research impingement. There are links on TW.
Sure you can take off from tennis and other stressful activity. A ball machine for you son?
You need to see a well qualified sports medicine Dr or other specialist and get a diagnosis as others have said.
So, to be clear. My P.E. teacher was wrong all this time? I cannot "just walk it off?"