Upper left back pain between shoulder blade an spine

So for a month now already after sleeping on some hard sofa for a few days I get this weird soreness between left shoulder blade and spine. What is interesting it flares up only when I straighten my back up and sort of drop my shoulders. It is quite deep, I can't feel it by touching a specific point. I tried doing some pressure massage with a tennis ball, and it does numb the soreness for like 30 minutes but then it comes back.
Its worse in the morning after sleep, and gets better through the day as I move. Did many back exercises and indeed it helps while doing them and shortly after, its kinda gone. but then again returns.
I do have a weak back though but I would want to know what kind of muscle from the upper back is responsible for dropping shoulders and straightening the back

Found online this gif relating to scapular setting, this exact movement is when I feel the soreness and discomfort on the left side between my shoulder blade and spine. If it gives some more clues.
https://i0.wp.com/post.greatist.com/...G_0.gif?w=1575

What muscle can it be? What should I do for the soreness to go away? Thank you