What Causes Tear In Rotator Cuff Leading To Shoulder Pain?

about a month and a half ago I woke up and I had pain going from the back of my shoulder down the back of my arm to my elbow. I got a injection for the back of my shoulder but still had the pain down my tricep to my elbow and it got worse when I sat down. Then about 3 weeks ago I noticed from my forearm down to my forefinger and thumb I would get that tingle sensation but only when I sit down. When I stand up it goes away.
Also. I had a MRI done in December of my shoulder and they found a tear in my rotator cuff and my bersa had gotten worse. I had surgery yesterday but they told my wife they didn't find anything how could that be?