People with IBS often have a rectum that is oversensitive to being filled, so sometimes gas causes problems and often there is a lot of urgency with BM's so they may come out faster than usual. Often the rectum acts like it is full to the breaking point even with just a small amount of something in there.Gas does not get from the intestines into the bladder under any normal situations. If you have a fistula (uncommon in IBS, common in some of the other GI illnesses like crohn's) you probably would get more than gas getting in the bladder as once you have an abnormal opening it generally lets everything through, not just gas.Some people have a similar sensitivity to filling of the bladder they have with filling of the rectum. The main medication for that is similar to some of the antispasmodics used to treat IBS and a lot of them will do both rectum and bladder as they don't know what they were approved for and a smooth muscle is a smooth muscle to them.