Formula is hard for babies to digest. You might try a milder or higher-quality formula that has stuff in it to be gentler on the baby's stomach. Also watch your own diet -- if you eat foods that produce gas, like cauliflower, the baby will feel it.
maybe her organism didn't get used to yet the formula and breastmilk together? how long has this been going on? if for a good few days, then better go consult a doctor:)