Apples are absolutely notorious for causing gas in people with IBS, especially in juice or raw state (some people are okay with small amounts of applecause or apple pie--which are cooked). And Des is right about the bananas causing constipation.
Personally, all fruit juice gives me gas--with apple juice being the absolute worse. I have to be very sparing with my whole fruits as well; like I said, never any raw apples. And some people on here have a really hard time digesting lettuce.
Oh, and if you are having problems with constipation, a low-fat diet actually isn't very good for you, because it will usually make it worse. The worst constipated I have even been in my life was while I was on a low-fat diet. I went from having diarrhea frequently to staying home to take laxatives because I was so backed up. You need some fat in your diet to keep things moving along. "Good fats" are the best bet for your guts--olive oil, fish, nuts and moderate amounts of dairy (if you can tolerate dairy), especially yogurt.
This may seem very strange to you--eat much less fruit and more (carefully selected) fat--but what's "healthy" for normal people doesn't apply to people with IBS. Sure, bad things (fried foods, trans fats, pizza) are bad for us, but good things (beans, apples) are generally bad for us as well. It kind of leaves you eating foods of indeterminate worth, like white rice, white bread and green tea. It's contrary to everything that's popular in dieting right now, but starches seem to be an IBSer's friend in most cases (provided you don't have Celiac's disease). I don't think I've ever heard anyone on here complain about potatoes, rice, bread, or pasta (sans sauce) bothering them.
Did they test you for bad bacteria? There's a relatively new test in the US in which you blow into a breathalyzer and it will determine if you have too much bad bacteria in your guts (because they produce certain types of gas that you don't normally exhale).