Could be a rotator problem (hopefully not a tear.....you would know because you would have pain), or you could have hurt where the pec muscle attaches. Reaching back like that could have done that for sure. I'll ask my wife, she use to be a practicing neuromuscular therapist.
She fixed my shoulder one time by working on the rotator and the pec. You may want to go get a neuromuscular clinical massage (it WILL hurt btw).
PVBDAWG