When a woman gets all cutesy and child-like when you interact with her, what is she feeling?

She's trying to be cute for you, and it's working :) Nothing wrong with that. The fact that she comes off as a b**** to others and is so "cutesy" to you sounds to me like she lets her guard down for you and is ok with being vulnerable to you at times. I think it also has a little something to do with nervousness but not in a bad way. It's rather endearing.