The key is to isolate what foods give you difficulty.  This means going, for a week or more, onto a very bland diet -- nothing spicy, nothing that causes gas (e.g. broccoli).  Then, once your body has adjusted, start taking foods away one by one for a couple of days.  Keep a food diary.  (See Healthcentral's here:  http://www.healthcentral.com/ibd/c/diaries).  Some times it's pretty easy to figure out -- something like milk or dairy products.  When you were younger you may not have had trouble digesting milk, but as you've gotten older it's harder.  Sometimes it takes awhile to figure out because it might be a combination of foods.  You should also check with a doctor or a specialist in bowel problems, like a gastroenterologist, to see if you need to be tested to make sure the IBS isn't something more serious.