Personally, I think it is the surgery itself that is causing the fatigue. The j-pouch surgery is much more complex and traumatic than the colectomy. Three weeks is very early in your recovery.
It could be the lomotil, but I would not think so. But, like Scott suggested, you can try reducing the dose and see if it helps.
Personally, I was fatigued for months post op. I did not take lomotil, but did take Imodium.
Jan