Why do I get lower back pain after a bowl movement?

Multiple causes. The causes are multiple, they include but are not limited to hemorrhoids, anal fissure, mass, spinal issues, prostate issues including infection. If this is a persistent issue, medical attention is warranted.
More information ? Why do you get back pain with a bowel movement? Are you having constipation? Are you bearing down to pass your stool? Where in your back are you having pain, and is it one sided? How long has this been going on? How would you describe the pain?