Why do i get lower back pain when i have not used the bathroom or when i am very bloated or gassy?

Referred pain: Often if you're lower colon is distended with gas or stool, you can feel pressure in your pelvis or low back. This is usually relieved with bowel movements. I recommend taking a stool softener or fiber supplement daily.

Pressure : When you do use the bathroom or when you are bloated and gassy, you do get back pain from pressure on nerves in lower back.