I often pull a muscle low between my shoulder blades, about 2 or 3 times a year. I can feel it building up after a period of intense exercise, and I furiously do stretches to keep it at bay, but it will "go" randomly doing something benign like picking up a toothbrush. I then have a period of 1 -2 days of intense pain, after which I will do a few hangs off a pullup bar and it then seems to reset and stop hurting.I've been told I have underdeveloped trapeziums (from where I am they look fine ), and have been doing some theraband exercises over the last few months so it has become less of an issue.
It might be worth taking a foam roller to your back to deal with the discomfort and also give your abs, pecs etc. a stretch, tightness here being a potential cause of over-exerted stuff in your upper back. Try lying on your back with the roller perpendicular to your spine, stick your arms up over your head and roll up and down a bit around the affected area. Bonus points for breathing during this process. It hurts so good.