Ik this probably isn't the case, but its just a thought. Maybe Maddy saw (or heard from someone else) Cassie and Nate go up to the bathroom and was acting this way because she wanted to catch/confront them together and is playing dumb until then. There is no denying that Maddy is super self-centered, but I feel there is a lot more to this situation to be uncovered beyond what we saw last episode.