I would bet good money that your back pain is caused by tight hamstrings. Before changing anything on your bike, try some gentle hamstring stretches before, during and after your rides. If you don't see any improvement, go to a fitter. You still might benefit from a fitting to optimize your set up, but to solve your most immediate problem, just stretch.
Live with intention. Walk to the edge. Listen hard. Practice wellness. Play with abandon. Laugh. Choose with no regret. Continue to learn. Appreciate your friends. Do what you love. Live as if this is all there is.
--Mary Anne Radmacher