Has she been there longer than you? Maybe last week was just a fluke in her behavior and her "real life" had problems and she was taking them out on you or something, broke up with her boyfriend or had someone else questioning her so she's paying back at the next "boss" that comes her way. See if she goes back to normal with the boss or if he has trouble with her and/or don't give her much to do, give other, more willing hands it to do so she has to come ask for work?