How long have you been playing DB? Is this back pain something new after a history of playing without problems? In any case, with all due respect to the helpful folks who have responded, this doesn't seem to be a problem that's very amenable to internet advice. Your bass may be too high, too low, it may be abdominal muscles, it may be back muscles, etc. I suggest that you have your playing posture evaluated by a teacher. If that corrects it, then fine. If not, then I suggest you seek face-to-face professional healthcare. FWIW, I have a history of lower back issues-- even had back surgery. My back was never bothered by playing the bass. I mention this to point out that everyone's different and that this is something that should be addressed in person.