Another thing I wanted to throw out there:
During my IBS research phase, I came across a website where people were posting that their IBS was triggered by (among other things) thickeners, especially carraggeenan. One guy posted that he had a particular brand and flavor of ice cream he always bought and ate with no problem, until one day he ate it and got a serious IBS attack, and realized the ingredients had changed and they started added carraggeenan. Other people posted about having similar issues with it.
A lot of coconut, almond, and rice milks and yogurts use this ingredient (or other thickeners like guar gum), so something to check out.