Yeah, that blank facial expression was definitely not a good sign. Mashiro has a lot of trauma, so I am not sure what exactly Makino's offer had activated, but she is not okay, that's for certain.
Mashiro's trauma is that she thinks nobody in the world likes her or needs her, except as a pussy for rent. She feels she has no use for anyone, unless it's for fucking.
She thought she was friends with Makino now, and the idea of having a friend made her all happy. But then, after hanging out with her for about a hour, Makino said 'Yeah ok I'm done with this shit, here are some bucks, bitch, let's get to humping!'
This is the first time, since the manga started, that I really want to kick Makino's ass.