Nuthatch- Check your seat height and handlebar height as well. Also when I get home I do a stretch for my back that eases the pain- other than that you'll need to do some strength training (my opinion).

Lay flat on the floor- bring your knees up to your chest- hold about 20 secs. do this 2-3 tiimes and your back should feel much better. Also you can do the same thing except bring one leg up to you knee- stretch and then the other one up. Alternate legs.

I will also stretch on long rides when I take a break. That seems to help as well!