What Causes Pain In My Stomach And Lower Back?

Dull uncomfortable pain at night - wakes me. It starts near my b button and gradually moves down the left side to my lower abdomen - then l feel the pain in the lower back also.Some times i feel the need for a bowel movement but unable to. I also feel some pressure in my lower left side.In the morning if I am able to have a bowel movement I have a great day.If I don t I have a very uncomfortable day.And it repeats the same symptoms.This has been going on for 3 months with more or less dull pain every night. I am very regular on a daily basis but after 3 days without a bowel movement I take a mild laxative - milk of magnesia - and I m ok for a few days. This process occurs about every 3-4 weeks. *Male age 64 - 5 10 excellent health - 185 lbs. - regular exercise - on blood pressure pills - crestor 5 .