Did you see a general practitioner?
General practitioners are great to control blood pressure, treat diabetes and take care of medical problems.
Very few general practitioners know how to diagnose a shoulder problem.
But it is also true that statistically, most shoulder problems represent an inflammatory condition (tendonitis, bursitis) that will respond to rest.
If this is indeed tendonitis, doing some rehab exercises like the thrower's ten (once all the pain has gone away) is less likely to result in an early recurrence once you return to tennis. http://www.muhlenberg.edu/pdf/main/a...throwers10.pdf
And later restarting your tennis with short hitting sessions without serving is a better way of easing back into the game.