You should enlist the help of your child's doctor. They can not only check her weight, but will also do a complete examination to look for other physical signs that she may have an eating disorder. They can also steer you in the right direction for therapy and support in your area if they also suspect an eating disorder.
Legal disclaimer: I am providing this general and basic information as a public service and my response to this question does not constitute a doctor-patient relationship.
For any additional information, advice, or specific concerns, please speak with your own physician. The information provided is current as of the date of the answer entry.