It might not necessarily be IBS. I caught a bug once I think in year 11 and found similar symptoms to what you're describing. It mysteriously disappeared after I started getting regular sleep, (like not waking up too early one morning, and then very late the next) and to eat roughly the same amount of food at a certain amount of time, so for example wake up, eat a ceral bar, eat another one at break, eat lunch, and as soon as I got into this routine, the problems dissolved. If you're having serious problems though, I suggest talking to your doctor about it and saying that you think it might not be IBS.
Hope it goes well!