Shouldn't be... You may have some constipation, which is much more common than folks think. Even if you're having a bowel movement a day, you can be constipated (the colon can expand and stretch a lot). If you have fever, blood in the stool, see a doctor. Otherwise take a softener like colace (docusate sodium) at least once a day. Do not take laxatives. Drink water! You should see a difference in a few days.
Have chronic low back pain. No diagnosis. Hurts worse when gassy or have bowel movement. It's not colon issues. What else could this be?
Diagnostic injection. Most back pain is either discogenic or facetogenic. That is either from your disc or the joints in your lower back. A less common cause is the sacroiliac joint. An MRI is useful if not done. After MRI history and physical exam diagnostic X-ray guided blocks can give you a diagnosis and lead to specific treatment.
Autonomic. This sounds like a sort of autonomic dysreflexia in which your nervous system has an abnormal amount of electrical signals going through it during the bowel movement. We see this type of issue with people who have spinal cord injury or traumatic brain injury. The back pain is possible as well as the rectum is just next to the tailbone that can send referred pain. Recommend see a pm;r physician.