Suggest Treatment For Aches On Upper Back Near Shoulder Blades

I have an ache across my upper back, mostly between my shoulder blades, but reaching out across the back of my shoulders. It began during the night, and was strong enough to wake me up from my sleep. One acetaminophen 650 mg tab was enough to wipe it out, but it resumed after 8 hours later, at which time I took a second tab of acetaminophen. I did work out last evening putting some stress on that part of my back, but It feels like it might be something else. What else could it be?