Hmmm. Shes about three months old now? How long has she been doing this for? Often, if babies are upset they take in air which makes them pass wind so this may not be the problem. Is she in pain? Does she go stiff as a board? Is this happening at nap times or is it doing any activity? Whats she like in the morning?
If it is colic due to wind pains, hopefully you should be nearly seeing the end of it. They generally say most babies are fine by 3 or 4 months.
If you think its due to formula, have you tried a "disposible" bottle? The ones with the bags that deflate instead of having the baby take in air? I used them until 3 months because I had a very colicy baby and they were very good.