Fibro does cause gut problems. I don't know if you can say you have IBS in addition to Fibro, or if you have Fibro with symptoms manifesting in the guts.
I guess you can say whichever you prefer. In either case, there's no real hard-and-fast cure.
My mother's doctor told her to try and eat organic; that preservatives, pesticides and misc. additives seem to make the fibro flare more often and worse when it comes. That might be something you try for the fibro and the gut problem.
Since you've noticed a direct correlation between worse stomach problems when you ride a certain bike and/or ride longer, then it would seem the obvious decision to either ride less, divide your rides up so they are less mileage at one time, but more frequent so you can get the same amount of miles in, or switch to a more leisurely bike. I myself have always wondered at people punishing themselves on those racing-style bikes with hard seats and staying bent over the handlebars all the time. Give me my replica 1940's-style bike with large, padded seat with shock absorbers and high handlebars anyday. 