Shoulder pain is a common complaint encountered in the primary care setting, and the majority of the time this is due to strain involving one or more of the four muscles comprising the rotator cuff. Typically this type of strain occurs in the setting of a change in exercise habits or after straining to lift a heavy object. Fortunately, most of these injuries resolve with conservative treatment, such as rest, ice/heat application, and non-steroidal anti-inflammatory medications (such as Ibuprofen). Muscular pain or rotator cuff tendonitis that is refractory to conservative treatment may be treated with injection of steroids into the joint space.
Warning signs of more serious pathology than a simple strain include redness or swelling around the shoulder joint, weakness/numbness/tingling in the shoulder or arm, or fevers. In your case, the new routine of playing tennis is the most likely culprit, and cessation/reduction of playing time would most likely accelerate the healing process. If you are adamant about continuing the tennis, then stretching before/after playing and the other conservative measures listed above may help alleviate your pain.
Please set up an appointment with your primary care physician
to discuss your shoulder pain. Good luck.