Hey ,
In addition to fatigue, another common problem with IBS patients is the inability to absorb nutrients properly. Personally, after years of tests and doctor's/hospital visits, I have found out that my digestive system processes food and drinks three times as quickly as a normal digestive system. This leaves less time for nutrient absorption, and therefore a lower nutrient absorption rate. This means you can consume the same amount of calories as a healthy gut, but unfortunately receive only a fraction of the nutrients. Your body needs these nutrients to grow/repair muscle and provide energy for even your easiest daily tasks. With a lack of energy from poor nutrient absorption, you are less likely to want to hop on a treadmill, or pick up your dumbbells throughout the day. When you do actually decide to pick up those weights, or go out for a run, because there is a lack of nutrients, your body will have a harder time keeping up with and recovering from strenuous workouts. The longer it takes you to recover, the longer it takes for you to build muscle, and ultimately maintain an active lifestyle.
My best suggestion is to get yourself on a routine with small achievable goals that you can easily stick with at first, then work your way up to bigger goals. Diet and exercise are all about consistency, so the more consistent you are with your workouts (and IBS controlled diet), the more success you will have. Progress may be a bit slower, and also harder to maintain, but with some consistency and dedication you can put yourself on track to see progress and maintain/achieve your fitness goals, even with the limitations IBS has so graciously bestowed upon you.