Maybe it's not so much the formula, but that she's teething and the sucking motion is making her gums hurt? My kids were sometimes slow to grasp a change so if you keep offering it to her she may eventually take it. Sometimes the type of sippy cup is an issue. My son was very picky about cups, until we tried the old-fashioned rock-n-roll tommee tippy. no spout. you do have to pay attention because they aren't spill proof and you would only want to give it to her in the high chair. My point is that changing recepticles might be causing a hard adjustment.
My dr. told us that we should not go to milk until 1 yr. of age. (now all dr.'s are different) Because cow's milk has nutrients in it that are harder to break down and sometimes it's harder because their digestive system is as mature. Not that it's not working, only that younger system, easier digestable things.
Try looking at what she was doing around the time she quit wanting to take it. Maybe you can determine if it's just teething, maybe it's too cold (if you refrigerate it)...something like that.