Could be a nerve if its travelling up the arm. It depends on if the pain is more or a sudden weakening/soreness/tenderness. Or if its a tweaked feling, liek a pinch or soemthing. Id say if it was anything bone related, which as you mentioned it not, youd of known.
I have chondromalacia in my right knee, its basically a weak tendon or lack of development in the tendons. Less cartiladge(sp?) than normal. That feel like it is weak and tender almost liek hanging on by nothing.
Id say go to a specialist and def. get it looked at soon.