May I ask what food you are offering her, does she have plenty of liquids to drink so she is hydrated . It would be a good idea to take her back to the doctor and get further tests done, is she at home with you each day , or in a day care ?
I agree with chuter15. Have your daughter seen immediately, though not necessarily at the ER. She'll probably need the kind of testing and evaluation that an ER doesn't perform. Take her to the ER with the intention of having her admitted to the hospital for a full workup and treatment, or just straight to your pediatrician with the same plan. Whatever gets your daughter the attention necessary to sort out what's going on and set in motion treatment to reverse it.
I suggest taking her to emergency. This has to be resolved. Children need the nutrients to grow properly, and it's crucial that they eat properly in their younger years.
I would also recommend therapy for her- maybe there's something going on with her that you don't know about. Good luck.