Cause - butterfly. ie. flexion/extension of pelvis. Ditto one week later, but not butterfly.
Big cause - too much sitting, not enough postural work, big imbalance in right musculature vs left in my low back.
Resolution - lying on my back with pillow in low back to reverse flexion position that I spasm into. Lots of trigger pointing of erector spinae, glutes and possibly QL (need to look it up!). Nurofen.
With hint of a twinge need to lie on my back flat on the floor and relax and it has eased once or twice - but no good after flare-up occurs!