that could be a cause of the cramping, definitely, but also just the fact that your colon is misshapen.
Just because you haven't been constipated doesn't mean you don't need fiber. An unhealthy colon with pockets, impactions, twisted areas, needs to be "smoothed out" and fiber does just that. You could have all that going on and never experience constipation. Constipation is mostly due to not enough bile produced to digest meals.
Fiber helps to exercise the colon, slough off old mucous that allows the unfriendlly bacteria to prolifreate, etc. Read Dr. Jensen's book and you'll soon know more than your doctor. ;)