Sharp pain in the shoulder blade

[mkh9]

Hi, I have been lifting weights and other gym exercises with a trainer and by myself for the last year and a half. Last two weeks I got a stabbing pain in the left shoulder blade (middle outer area). I have had a massage and rolled it with a Lacrosse ball and that didn't work. I then found that my front left pec muscle hurts to the touch. I also had trouble moving my left arm backwards. My trainer is a master trainer so he knows a lot about injuries and things they are connected. But, I also noticed that when I press on the pec muscle that it takes the pain away. so is this being caused by the tight pec muscle? I can still work  out and it doesn't seem to bother my back any. Resting and working out don't make it better or worse. It doesn't feel like a pinched nerve.
thanks,
mkh9