Babies at this age are still developing their digestive enzyemes, as you are experiencing all formulas are not created equal. Have you discussed with your doctor about adding probioitics to your infants diet to help boost the good bacteria in her digestive track and ease digestion? Here is a little blurb on probiotics for infants: http://www.examiner.com/x-7158-Chicago-Family-Health-Exam...
Babies born via csection tend to not get the good bacteria as they would through the birth canal. There is a reason for and it is not normal.
You can try an organic formula or an easier to digest such as Nutragimen (which is what I supplemented while I nursed). I also added probiotics to the formula, many companies are doing this now because they are finding probiotics in breastmilk and formula companies try very hard to mimick and create what is in breastmilk.
If you decide to add probioitcs, FloraBaby is a wonderful brand just for babies/toddlers under 4yrs old.
Hope the info helps.
J. W. MPH
Maternal and Child Health Educator