i've been suffering from the same thing for the past decade

it's the PC.. you usually use your right arm for most of the things on the keyboard and most importantlly, you use your right hand to move the mouse..
the anchor muscles are on the left side, behind the left shoulder blade and also just behind the collarbone...
there are some ways to mitigate the ache.. learn how to use the keyboard instead of the mouse... stretch ever half hour... stuff like that..
and.. i volunteer my services as a masseuse for your shoulder

