What could be the cause of stiffness and discomfort between my shoulder blades when lying down?

This began about three mornings ago when I was awakening, and ONLY happens then, not when I am walking around, nor sitting upright; not even when I first lie-down to sleep! It is a stiffness, soreness and discomfort that seems to be localized from my left shoulder blade (underneath it) and across towards my spine. This is not a neck, mid-to-lower back issue. I can't rub anything on it because it is the part of my back that is almost impossible to reach! I am thinking Chiropractor, but can't really afford him right now. I'm in my mid-fifties and generally active and healthy. Any home remedies? HELP!