The gut lining is shed off or exfoliated and rebuilt completely about every 10 days the way I understand it. Too much iron can speed up this process to our detriment. Shed off enough stomach lining cells , fast enough and deep enough, and down to the blood vessels, and then you have a bleeding ulcer. It was also found that adding iron salts to a culture of Helicobactor pyloric enhanced the growth of this bacterium, and several others in the same experiments. All of these organisms tested were unfriendly to the gut. - and to the body.
Iron plays a very important role with the pancreas.
The iron causing this aspect of IBS would have to be absorbed and deposited in these pancreatic cells mentioned and in excess to cause these problems. You will be told that if there is excess iron in the lumen of the gut that it will not hurt you for you will not absorb it unless you need it. The fact is, the more iron there is in the diet, the more iron that will be absorbed. Once absorbed, the body does not lose iron unless there is bleeding - one milligram a day excepted. As explained, if not absorbed, the iron is still in the gut until it passes through that most posterior aperture causing problems all the way along the entire length of the gut.