Have you injured yourself recently? Fallen or pulled a muscle? Maybe you have a muscle that hasn't fully healed yet.

I've never experienced any back pain in climbing. Sometimes nerves get pinched in the hip/upper thigh area and makes your back hurt like crazy. A friend of mine had a ruptured disc in his back and a pinched nerve in his leg. Both hurt in his lower back area.

Does it feel better once you quit climbing?