It could be the food. Is she doing well on it otherwise? Like no diarrhea or itchy skin. If she is doing well other than the gas, then I would add probiotics like natures farmacy http://www.naturesfarmacy.com/store/details.php?prodId=197 to her food. It is hard to find a good food that works for them.
Besides the food, the only other cause I can think of would be stress? Has your/their situation changed at all so the routine is different? If not, I'd try a probiotic and if that doesn't help, then I'd consider changing foods (but keep the probiotic...those are good no matter what you feed).