A.R.
I don't know anything about formula feeding amts since I breastfed, but I know with breastfed babies who are sleepy, it means that they are conserving energy/calories and they need to eat MORE.
I would call your pediatrician if you are worried- and it sounds like you are. If this is a big change for her and her eating amt has DROPPED then it's not a growth spurt- she'd be eating MORE, not LESS.
Just my thoughts....
M.