I have read that some injuries, including some shoulder injuries, because of the lack of nerves in the injured tissue, may progress for some time without much or any pain.
QUOTE from an earlier reply
This Todd Ellenbecker video is the best detailed description of shoulder issues that I have found. At minute 8 he describes the orientation to hold your shoulder in serving to minimize the risk of impingement (similar to the Jim McLennan video advice in Reply #3).
There are many recent shoulder injury discussions in the Health & Fitness Forum. Use to inform yourself but it is a very risky approach to diagnose or treat yourself based on the internet.
I have read that with shoulder pain you can be doing damage without much pain so take it very seriously. Some swelling
may also be present that might make any mechanics painful.
Best to stop and see a Dr.
For mechanics you need to study the serve, video your own serve (60p fps video can be useful but high speed video/fast shutter is excellent). A well qualified instructor who can coach the serve is a good idea. In any case, you should especially not be experimenting with your serve if injured. Can one bad motion tear something? "