Why do i get bad lower back pain when i need a bowel movement. ?

Colon spasm: I suspect that you are experiencing "referred pain" from your colon, which will go into a bit of a spasm to expel its contents when it is time for you to defecate. If this is a new problem for you, it might be wise to check with your doctor to ensure it isn't something else. You might also see if an OTC stool softener like Docusate might relieve the symptom somewhat.

Unity of structure: Your body is one unit. Your intestines hang from your lower back. There is overlap in nerve supply. People who are chronically constipated can easily get back pain. As precaution, If you have other medical problems, or a family history of colon cancer, get checked out by a doc in person. Make sure you've had a recent gyn exam too. I hope you feel better.