People with IBS run the same gamut of weight problems that people without IBS do - some are trying to lose, some are trying to gain, and everything in between.
In general there really anything about the physiology of IBS that would cause you to lose or gain weight. The only thing you might notice is that *some* people with IBS, when they get really really bad (usually before finding this diet), find it difficult to eat much of anything, and therefore lose a lot of weight, often very unhealthily. If that's not you, consider yourself lucky!
As I said, some people with IBS are overweight (just as some are at healthy weights and some are underweight), but there really isn't anything, as far as I know, that links being at any particularly weight and having IBS.
There are no excercises that are specific for IBS or better or worse for it -- well, all forms of exercise are good for IBS in general (staying active etc.). Just do whatever you like best!