There are only three things I can think of when abducting the arm in "computer mouse" position.

Serratus problem
Trapezius issue (upper, mid traps)
Supraspinatus -
Deltoid

I am thinking it shouldn't be Long Thoracic Nerve/Serratus as if we are sitting with arms very slightly abducted, the Serratus requirement should be minimized, correct?

Is your pain area the same as I linked?
http://www.physio-pedia.com/File:Self_Exercise.png