I'm not sure...

I'm not sure, but my guess would be that your nerves are playing a big part in what your small intestine and sphincter muscles are doing throughout the day. My best bowel movement of the day is always a morning one, and I've always assumed that the reason for that was that I was sleeping for hours prior to that bowel movement, so my body was calm and did what it needed to do. After I wake up and start my day - I'm moving around, my nerves are awake, I'm eating, etc. etc. All of this gives my body less of a chance to get the bowel movement formed, not to mention gravity working against me. I'm sure there is a much more scientific explanation that someone after me will articulate. Just know that you're not alone! I have the same trouble when I'm in a flare. I never really thought to get an official answer from the doc though. Best of luck to you, and like I said - I'm sure someone will be along soon with a more researched answer!