Why does Anne's mother think she's sick?

Anne's mother believed her flushed face and warm skin were symptoms of a fever or forthcoming illness, but we all know they weren't. Her symptoms were a direct result of her crush on Peter..... teenage hormones.

Mrs. Frank (quietly). Anne, dear . . . your hair. (She feels ANNE’s forehead.) You’re warm. Are you feeling all right? 

Anne. Please, Mother. (She goes over to the center table, slipping into her shoes.) 

Mrs. Frank (following her). You haven’t a fever, have you?