I have some sporadic lower back pain. Not sure what causes it --- one theory is a bad fall off a bike a few yrs ago, and subsequent tension --- other theory is standing calf raises with a heavily-loaded smith machine bar (my hunch is that can compress the spine). Whatever. I went to my doc he could find nothing obviously wrong; sent me to a chiropractor. That didnt help.
Then recently read an article about this which suggested: 1- standing against the wall, with a foam roller b/w one side of your lower back and the wall. Squat up and down, pushing very hard into the roller, on first one, then the other, side. 2- lie face up, bend knees bringing them up to chest, then grab them and hold tight til you feel a good stretch in the lower back.
Might've helped me a bit, so you might try it too. Good luck; bad back pain is the WORST.