I was diagnosed over 10yrs ago with IBS. The doctors werent helpful in the slightest in pointing out what the problem could be.
Avoiding all aspects of dairy (inc eggs) has dramatically improved my situation. Its hard to do at first but once you look into things and get your shopping routine going it soon becomes easy - especially today as food stuffs have much better labelling.
The problem is now though that if I touch the slightest bit of dairy I know about it and a normal portion of something dairy results in IBS rearing its head up to 3 days later.
It is believed that the Dairy/Eggs things results from gluten clogging and disrupting parts of the bowel that would normally digest this foodstuff.
Interestingly humans are the only mammals post infancy that deliberately choose to continue consuming dairy products.
I met a guy once that didn't eat Dairy (it came about because I was putting orange juice on my cereal!) - when I asked why he stated it was to avoid the radioactive contamination that the hill sheep have from Chernobyl fallout - maybe hes right and maybe thats why our bodies, in greater numbers than ever before, are choosing to reject this foodstuff.