In reply to GarethSL:
I have had this problem and as a result of working a traverse section on a hard boulder problem. I found it was my general lack of flexibility in my legs and hips, this meant my lower and mid back had to work harder to assume the body positions.
To address the low lying pain, try cat stretch, sitting in a chair facing forwards and gently twisting left to right.
Gently work up doing twists and back bends. Finally if you are feeling better work on addressing tight muscles in your legs, hips and back. Google "becoming a supple leopard" book. Crap name but the contents is phenomenal and is a bit of a go to reference book for avoiding injury and dodging handing over large sums of cash to therapists.
If all else fails go see a qualified medical professional, but be mindful that if you can walk, care for yourself and work, support will be limited.