I had a proper messed up back a few months back following a rather embarrasssing testicle incident in tesco's, with the injury at its worst it was only my time out on the bike which made me totally free of pain, so i rode a lots.Dr encouraged it also.
As it healed a bit i did find that post ride i would have some lower back pain,
what was also pointed out to me is that cycling shortens the tendons and builds thigh muscle,which pulls down on the lumbar region, they put it in much better fancy smancy medical speak but you get my point, the fact was brought home to me also when they asked me to touch my toes, and while although relatively fit i was an inch( perhaps 2) away from toes because of overtight things in backs of legs.
I also find with a fully loaded pack and loose straps my back gets a bit jippy, if you have linked back pain with cycling post ride, check your bike set up