See this picture well see where the word scapula is and Humerus? That is where I feel the most pain. It radiates down to the arm and even up where my socket is.
Impingement is where the supraspinatus muscle of the rotator gets pinched up near where is says acromion. When the arm is lifted out to the side, the space between the humerus and the acromion gets narrow...and if you have arthritic thickening of the acromion, the space gets even more narrow and can pinch the supraspinatus muscle.
However, where you are describing the pain, it is more likely the subscapularis muscle that is involved. It is one of the four rotator cuff muscles. It turns the arm inward.
I bet you can't reach back to fasten your bra without pain.