just this week i injured my back, also mid-back, lil lower tho, on the right side along the spine. been a "knot"/discomfort there for long time, spanning back to before I trained. had done a heavy workout wed. (including deads), no problems/unusual pain etc etc. thurs. was at work and simply did an upward reach stretch and held for like 30 sec. and then lowered my arms n took a few steps and wham- my back seized up hard, it was gripping n hard to breathe, n move well. next morning couldnt straighten my back upon rolling outta bed. went to the doc n he said i over-stretched the problem area (activating the stretch-reflex/contraction, causing the already tight musculature to seize). had to take a couple days off work, n much worse- have to avoid training or do things that avoid much back involvement (good luck right?). i have never had something like this happen but the nagging pain like you describe was a warning sign i ignored for a long time. not to say yours n mine are the same but be careful man, best wishes for health/healing
