Google the "piriformis." Off and on I've had sudden lower back pain that no amount of stretching helped. Cycling friends were talking about piriformis issues and I did some poking around. I found that when I had back pain that bent me over nearly double I could get relief by finding the right spot on my butt and pressing it. It's buried deep and takes some searching but you'll know when you find it. I use a foam roller for my hamstrings and that also helps. Back pain sucks, hope this helps.