Yes she is right. Current culture compulse girl like you to be slim as it is felt that slim is only beautiful. But that is not so in reality. Many girls are facing problem of anorexia nervosa an eating disorder in which person feels if she/ he would eat , she/he becomes overweight and look ugly. So they avoid eating. Even they take food less than the requirement of body. So one psychological belief cause physical problem like malnutrition and weight loss. Some girls faces stopping menstruation or scanty menses for one day only. So just follow your mom advice. Take meal properly and consult doctor once at least that you are ok physically to assure yourself and your mom.