Everytime I Eat My Abdomen Fills With Gas. After I

Everytime I eat my abdomen fills with gas. After I can get rid of some of the gas my back still hurts in the area where the colon would be. My doc said that I have severe constipation but it has gone on for a very long time. Weeks. He put me on Milk of Magnesa and that helps but I have the same things happen I eat. Sometimes I can eat a food and then the next time it will make me hurt like crazy!