The rotator cuff "S.I.T.S." (acronym for all the muscles it includes) don't include the deltoid. The deltoid is important in hitting the serve and forehand and should be strengthened as well to avoid injury.
The pain you describe could be any of many muscles in the shoulder. Sounds like lateral/anterior insertion of deltoid on humerous but could be any number of things. Go to the Dr.
See earlier post on shoulder problems/serving in this same section and see an Orthopeadic Doctor IMMEDIATELY.