I have IBS and a severe intolerance to all forms of soy. Most breads do contain soybean oil, and some have soy flour. Until I completely gave up soy, I had similar symptoms.
I think that IBS is usually connected to some type of intolorence and yes, i don't see why you couldn't have some type of perservative intolorence. avoiding those foods that contain this would be the way to go. Do you have anxiety? the reason i'm asking is it seems like there is a connection with stomach issues and anxiety. whether it is the anxiety that causes the stomach problems or visa versa, somehow they go hand in hand.
Prolly shoulda mentioned this, I do have a prescription for IBS medication and was full of joy when i got it only to be disappointed with no help or relief from this problem. I have tried a few IBS medications and nothing seems to work.
I have IBS, CFS, fibromyalgia, and neurally mediated hypotension and have found that avoiding all gluten products vastly decreases pain (including stomach) and improves energy levels. I can have a little here and there, but if I eat multiple items in a day or even, more than once a week my stomach and my body reminds me why I avoid it. It is common for people with these conditions to have a gluten intolerance.
Bread is supposed to be very hard to digest. See a gastro MD and they can give you some meds to take care of the IBS.