I played semipro soccer 10 years ago, halted by kneecap injury, but besides that no injuries, only time I heard my back crack was when defending a friend from a beat down 2-3 really big guys (club security enforcers) pinned me down and kicked the shit outta me, they couldn't drop me for about 30 seconds while hanging down from my neck/back and when they did I heard the crack but I don't recall the pain lasting more than 2-3 days. That was 10 years ago as well. The pain is focused in the middle of my back, constant spurts of pain without pause, when I get up and stretch a bit I feel a little relief... I don't feel any weird sensations on my legs... I recently started working in an office after 2 years and the pain started after the 2nd week... been a month now, I'm guessing related to not being used to sitting down plus a bad posture