Capillaria or hairworms are small nematode worms that inhabit the stomachs and can interfere with digestion. Heavy metal poisoning. Many of these conditions require microscopic examination of the faeces for diagnosis. Special stains may be necessary. A physical examination of the bird will probably reveal that she is underweight.
Birds can conceal that they are unwell until they are very sick and debilitated.
It would be advisable to catch her and examine her chest for muscle wasting. A healthy bird should have pectoral muscles like a roast chicken. She will be eating the sunflower and canary seed from her mixed seed mix as these are high in fat and will give her more energy per unit volume, as she is not extracting energy from her food efficiently due to the digestive problems she is experiencing. Millets are generally lower in fat content, which is why she is avoiding them. Regarding worming, you should consult with your avian veterinarian for the most appropriate worming regime for your situation.