Or if she asks a stupid obvious qustion like "What's bothering you?"
Just look at her, not angry, just look at her... let her READ in your eyes taht she's been a selfish bitch for weeks and you aren't gonna tolerate it anymore...
She will likley walk away in shame... or continue to play dumb.. I've personally seen both responses.. both are ok results
My favorite at points like that is to just stare at her, incredulously, and shake your head and slowly say ...