Can't say for sure, but a lot of middle back/neck pain can be caused by sleeping problems... pillow height, tweaking your neck while sleeping etc. You don't notice the pain or problem until you do a certain motion then it's 'ahhhh shiiit.'
It used to happen to me all the time. it's 'deep' in the middle of the back and doesn't go away for a few days. One good stretch to see if it might be a neck problem (or maybe help stretch out your hurting muscle) is this:
-stand up or sit with a straight back, good posture. Try to relax your shoulders as much as possible, let your arms really hang down.
-slowly lower your head so that your chin is touching your chest. don't crank it right down, the key is slow and loose to keep any major muscle tightening from happening. try to look at your chest; if it starts to feel really tight look back up, relax and move your shoulders, and start over.
-breathe out; if your chin touches your chest you're good.
-slowly and carefully turn your head from the center (still touching your chest) 90 degrees to one side, while trying to keep your chin touching your chest; the goal is to get your chin touching your shoulder. you need to BE CAREFUL and breathe in and out and stop if it gets uncomfortable; you'll hear all kinds of cracking and tweaking and feel all sorts of muscles screaming at you. if you are relaxed enough and can get a deep enough stretch you should feel the upper middle of your back and lower neck really stretching.
this stretch can actually cause neck pain and muscle tweaks, so be careful... i read up on it after constantly pulling muscles in my neck and back from sleeping weird. but if it is just a chronic pulled muscle and you need to stretch the area, give it a shot.
oh... 2 rounds a day 6 days a week? take a few off man. get fresh.