Pressure underneath my left rib before bowel movement. Perhaps colon or intestinal obstruction.

For about three years now, I've been feeling pressure underneath my rib now and It usually happens right before I have a bowel movement. Actually I know when I'm going to have a bowel movement later that day because of that pressure underneath my left rib. If anyone has any idea of what this is I would be very appreciative of your help, Thank you.