Strange has indeed done something impulsive in the sense that he knocked back some concentrated madness and is hallucinating a teensy bit right now. He opens the door, looks at McGonagall, and then the mouse that provided the madness and he can still feel running around his brain just freaks the hell out.

Strange stumbles backwards, tripping over a book he threw on the ground and wiping out in the process. Because this woman is a cat. She's simultaneously a woman and a cat, not the woman who became a cat who gifted him the madness in the first place. A cat and a woman that exists in the same space.

This is amazing and terrifying all at the same time: from his position on the floor, Strange looks up at McGonagall with an expression that's half awe, half outright fear.