I am experiencing pain in my left arm. I am right arm dominant. The pain is mostly muscular and affects every muscle from the bottom of the shoulder to the wrist. It is not appear to be rotator cuff as the pain is not in my shoulder, I have no problem sleeping on it and can raise my arm easily over my head. Because I see a neurosurgeon for a small brain lesion, test have been performed, and it is not neurological. The pain is most severe when trying to carry anything with a bent arm....even trying to carry a cup of coffee can be excruciating. Went to an orthopedic for a foot issue, and he diagnosed rotator cuff, and I have been to a couple of physical therapy sessions. Not going back because I'm convinced it is not my rotator cuff. When I try to carry my 2 yr old grandson, the arm simply fails under the pain. I am a 56 yr. old. white female in good health, weighing 112 lbs.
Remember, the rotator cuff is make up of 4 different parts, so you can tear or injure one and still have range of motion with minimal loss. However, when just one is damaged or torn it can be very painful(especially, when that move is tied to the particular damaged part of the cuff...such as picking something up with a bend in your elbow..), and the pain can radiate down the arm. You may also have some arthritis built up in the A.C. joint that the rotator cuff works around causing significant pain. If you are trying to avoid surgery then you can get cortisone injections right into the joint for 5-10 months of relieve. Honestly, you best bet is getting an MRI if that is within your means to see where the damage is and have it surgically fixed if needed. Based on the movement you say causes the most pain points to the infraspinatus, the second tendon of the rotator cuff, with some combined arthritis in the AC joint. Best part is that it can be repaired.
Best of luck