I Have Been Experiencing Bouts Of Stomach Cramps And Frequent

I have been experiencing bouts of stomach cramps and frequent bowel movements and diarhea. It usually begins about 1 1/2 hours after I have eaten lunch. I am feeling well otherwise. This has happened off and on for about a month, usually about a week between episodes. I am living on Imodium in order to finish my day. There seem to be a lot of conditions that might possibly the culprit. How do I know where to start?