I've experienced all sorts of back pain, all with various different reasons behind them. A constant lock on the belt could be the culprit, but I wouldn't put the mess bag out of the question either. Most of my pain was in the shoulder, I was also using my mess bag as a walking around bag too, which was stupid. Bottom line, no matter how comfortable and ergonomic mess bags are, they were made to carry cargo while on a bike and everything else secondary. And considering how much time i spend off of my bike and how many other people I see with mess bags who aren't even cyclist, that's a whole lot of people who are going to be having back and shoulder problems in the future. OP, seriously, if you're going to be using this thing as walking around bag, not just for this trip but afterwards as well, do yourself a favor and just get camera sling pack. You can find decent ones for under 80$, which just about as much as you got your TB2 for i think.