Maybe someone else will have more insight, but when I see vague symptoms like this and do not know the cause (which is most of the time), I make sure they stay hydrated by dipping the beak in water periodically. Even for adult birds. Often they get over whatever it was that was ailing them. My personal theory is that dehydration often kills birds that could otherwise have recovered. They are small animals which lose water rapidly.
Continue to watch for additional symptoms, such as diarrhea, difficulty breathing, etc, that might lead you towards a more specific diagnosis and treatment.