Under Arm Muscle??!

Hmm, well at first I thought I simply had tight triceps as that is how it felt just walking around. Now on closer inspection i have realised that my tender spot is like my underarm. It is hard to explain where, but if you raised your left arm straight up...it would be the 'outside', left hand side of the underarm. The muscle that runs up into the outside of the arm I guess....
I was wondering what muscle this was, because It¡??s damn sore

To me it sounds like youre talking about your lats. Your "latissimus dorsi" run from under your armpits down your back. When developed they give you that nice wide appearance from the rear. Here is a pic, in red obviously:



This musclegroup is stressed with pulldowns, rows, pullovers, etc.

nice to know the "full name"