Why does constipation cause really bad lower back pain?

Doctor can examine: Patients who complain of constipation usually don't say they have bad back pain that comes with it. Thus, it would be a good idea to get a primary care doctor's evaluation to see if something else is going on with the back. One should write down details of symptoms, such as when back pain occurs (is it only during pushing out stool, is it only after pooping, or only before pooping, etc...).