I don't know how much of the conversation you caught op, but isn't it possible she was referring to something completely unrelated to you? Maybe she had a weekend gig where she wasn't allowed to leave the house or eat their food? I don't know, I wasn't there but that seems plausible. Or maybe she was retelling something another nanny told her. Or maybe your nanny is a jerk. My guess is she was either referring to you not offering to pick up a cheeseburger when you bought one or she wasn't referring to you at all.