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:)
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.