You decide the best way to solve the problem is just to ask for yourself. You walk back up to the bathroom door, this time without entering, and just give the door a firm knock. “Hey! Are you my new roommate?” you ask her outright.
After just a moment, the girl inside responds. “W-What? Oh, uh, yeah! Hope it's not awkward for you, living with a girl!”
That's weird. She is your new roommate—or at least, that's what she claims. Do you still doubt her words?