My sister has IBS-C and she says that when she is bloated and has more gas she can't do any type of movements of foreward bend type and crunches would fit in this category I guess.
If you get gas or feel like the gas is trapped and doesn't want to come out though I recommend trying the wind relieving exercises in yoga, you can do it with right leg up first and then left leg or both legs at a time and you can try to wiggle gently from side to side or and back and forth. (See the Yoga for IBS link for picture of wind relieving exercise with one leg at a time.) These exercises really work for us anyway.