The week up to the race I cut out all processed foods and just keep to my healthy diet with no focus on decreasing fiber intake
This could be worth looking into. Is cuttingout processed foods something you do regular, or is this generally only during race week? If you’re cutting out processed foods, I assume that you are increaseing your intake of whole foods like grains, veggies, fruits, and meat. All except the meat will increase your fiber intake and can lead to some temporary gi distress regardless of doing a full IM. This is normal but is actually healthy- your body does adjust to greater fiber if you keep it up