it is made worse if my camelback is particularly heavy.
Try a Wingnut pack that sits lower...helped me.
shorting reach and raising bars can actually make things worse, as you try to compensate to make the bike work for you, and in some cases you need to do the opposite (make the reach longer/lower bars) but impossible to say with out an expert looking at you on the bike or a bike fit.
+ 1...I'm very much low bars, long reach. As others have said, it's a very personal thing and was very much trial and error for me to find the right position. Reach, saddle height/angle and bar height can all have an effect and my setup only has to be out by a few mm and I suffer lower back pain within the first half an hour of riding, yet setup correctly and I can cycle for 10hours+.