Funny that this would happen right after gall bladder surgery. Maybe it was a 'coinkydink!' (:
Fatigue and low energy levels COULD be symptoms of IBS... but they are not on the top of the list exactly. IBS symptoms are most always directly related to intestinal problems... if a person has extreme diarrhea or constipation...this can lead to many OTHER related symptoms...including the ones you describe.
Sounds more like you might have Chronic Fatigue Syndrome which has a multitude of 'causes'... BUT!--I believe it CAN be managed. I will get bouts of this because I have fibromyalgia...but not to the point some people are afflicted with it.
Brain fog is usually caused by the body having to fight off and succumbing to toxins. A good Colon Cleanse might be the answer.
Also...what helps me is Vitamin B-50's ... a good grade of these vitimins.
I am starting to do the Flaxseed Oil and Cottage Cheese thingy because I have friends who just RAVE that they have so much more energy now! ... its not just to cure cancer.
I hope some of my rambling helps you! (: Best wishes to you.