Thanks for those links. I will try to exercise it with eccentrics with low enough weight so it doesn't hurt.
I have tried feeling around my chest and shoulder to massage it. I can't really find any place where it feels tender though. It feels the same as the other side. Maybe I am just having trouble getting underneath the pec major. I have not done any ART, not really sure where I would do it.
One other thing, sometimes if I am sitting in a chair with armrests which elevate my shoulders, it hurts the left one when I get up, even though the shoulder is just being depressed passively by gravity. Maybe there is still some eccentric contraction I'm just not aware of though.