Where I work, you can request a special chair that supports your lower & upper back. You have to get a physical in the medical department first, but they'll buy one & have it delivered to your office door if you are experiencing lots of back pain. Have you thought about asking your manager/boss for something like that?
Another one of my coworkers will gather a group around lunch time to stretch and just get up & move (since a lot of us eat & work at the same time!) My favorite exercises for making my back feel better look like this:
Start here:
And then in one fluid motion: lift up into all fours, arch your back, lower to the ground, lift your head up, and the end position looks like this:
It always makes me feel better!
Good luck!