Maybe your LO has acid reflux? Mine does, and before we got him on a prescription for it, he could be pretty fussy during feedings and spit up quite a bit afterward. Or, maybe your LO has an allergy, and the formula that you give her irritates it.
My LO is primarily drinking breastmilk, but I do supplement some with formula. Before he was diagnosed with acid reflux, I experimented with several types of formula to see if any of them would make him more comfortable and spit up less. I found that Neutramigen helped some (it's a little pricey, but it may be worth it). Since my LO has acid reflux, I have to give him smaller amounts more often. At 17 weeks old, he drinks 3-3.5 oz every 2 hours during the day, and every 4 hours overnight.