A couple of comments. I also took Amyltriptaline (10 mg... a minuscule dose) for my fibromyalgia symptoms, of which GI stuff was a part. It really worked and I only took it for 3 months. It can cause weight gain, so I was really vigilant with my eating and exercise. I also took the mindfulness meditation course at the same time and continue to practice some of it; I use it extensively with my clients. It really works.
But, I think there are 2 issues here. I do have the IBS symptoms, but they wax and wane according to my diet and stress. I now know what sets it off and sometimes I am willing to pay the price. I can have one cup of coffee with no issues, but sometimes I just want two! I don't eat much dairy except cheese, but I do love spicy food. I don't eat onions... that's an automatic set off.
However, I also have the same thing Emily and Mimi describe and so does my DH. We call it "the dreads." It's a feeling of dread or nervousness, accompanied by at least one trip to the bathroom before a ride, but for me, it often is 2-3 trips, before a ride. Even if the ride starts in the PM and I have gone in the morning, i have to go. My older son described the same exact thing happening to him before a ride, too! I am fine once I am on the bike.