All good recommendations but after you see the doctor, get multiple cortisone shots, physical therapy, stretch bands, etc. and still can't throw a ball (as I experienced). Get yourself a hard rubber ball(lacrosse or large super ball), a wall and read
Http://www.mypressureproducts.com/pa...ence_chart.htm.
Start working the trigger points several times a day with the ball. Once the trigger point pain resides and your arm feels better buy a speed chain and start exercising the rotator muscles. Figure eights, circles, tennis backhands, overhead throws. Your rotator muscles will be stronger than ever.