Originally Posted by Mike94CivicEX

Hmm...middle left back? If you said you were stretching out while doing it I would say you sprained or pulled a muscle or tendon attached to the tubercle of the rib or the scapula, not really sure but I would bet it's a muscle/tendon pull/sprain. Nothing to get worked up about, but it will feel a little worse the next day.
Don't take my word for it, go to the doctors and get it diagnosed and get some pain relievers for it, keep me posted on what it was.