I Am Having Severe Pain In My Very Lower Back

I am having severe pain in my very lower back when I walk and it radiates to my legs really strongly when I have a bowel movement. It is getting worse over the last couple weeks and is getting to the point I can't hardly walk and dread having a bowel movement. What could most likely be causing this pain? And I do have a nerve impingement at L4 pinching the L4 nerve root.