I have also had problems with fiber-mostly bloating and constipation (sorry if that's tmi). I tried fiber powders that you mix with water and ended up really sick. Lettuces, beans, whole wheat breads are ok, but some cereals make my tummy issues worse, so I eat cereal only 2 or 3 times a week or less.
My only advice is to take a few days off the high fiber stuff, then add things back gradually until you figure out how much you can eat without getting the bad symptoms.
Also, might try to exclude the gluten-containing fibers and see if that helps. I cut the gluten for a couple months and it didn't make a difference, but I know that gluten can be a big issue for some.