I won't say it is a pain. It is rather dull pressure in the back. As you climb overhang, your hyper extension of the back causes. I found a French site with great stretches I was describing in the previous article.
Translated from definition section
The word simply defined back pain pain in the lumbar region. That is to say, the bottom of the spine. Localization of the lumbar region of low back pain are many climbers, both in the type of pain in their causes. From the surface to depth, the different structures that can generate back pain are: • the muscles of the lumbar region, which may spasm, or because of the effort, either to protect the underlying structures, · intervertebral ligaments, which may suffer a prolonged stretch, · joints which can be worn when one speaks of osteoarthritis, · intervertebral discs, which can also be worn, crushed, or submit an overhang or a hernia, · meninges, envelopes the spinal cord, which may undergo various stresses (tensions , pressures).You can use Google translate and it does adequate job. There are videos of stretches. It is great.