She may be a weak or failure to thrive chick, but I would treat her with vitamins and try to help her survive. Walking backward is a neurological symptom. Human vitamin E 400 IU softgels daily may be helpful, since she needs a little more than what is in the PND.
I would feed her the whole egg, just mix it well with a fork. Make some mushy chick feed and feed that as well. Is she having any trouble walking? Give her 2 drops of undiluted NutriDrench twice a day for a few days. Then dip her beak in water as often as you can all day long. Check her for any...