You have developed some temporary pain with what is called a Valsalva maneuver. This occurs when you have a bowel movement. You have to increase thoracic pressure (chest) which in turn increases intraabdominal pressure and finally this increases pressure in the lumbar spinal canal. This increased spinal canal pressure will aggravate previously irritated structures like a nerve root. If the pain is not impairing, I would try to ignore it.
Dr. Corenman
