This doesn't happen very often, but I would guess at least once a month. It's kind of hard to explain the feeling. It doesn't feel like a pulled muscle as I've felt what a pulled hamstring and calf felt like. It feels kind of deep and to the side of my spine and level with my shoulder. It's not always on the same side, but I'd say it's in the same location every time (left or right), but just 1 side at a time. I'd also like to mention I had heart surgery when I was a baby, if I remember correctly it was to fix a valve. I have a scar on my back that runs along my shoulder blade (never realized they did heart surgery from the back too).