Do you check the mill date on each bag of feed before you buy it? Some vitamins are lost over time after milling, definitely. I buy feed that's within one month of milling, and use it up within another month. The feed store folks say the feed is good for six months to a year, but IMO that's just too long.
There are individuals who need more than the 'normal' amount of a particular vitamin, and there are individuals who just want to eat too many goodies, rather than their regular balanced feed. Either way, this could be your bird, and adding a vitamin supplement may help her. And she can't do it on sunflower seeds!
I hope she's able to recover.
Mary