Hard to say from the description and pics. Look the bird over the entire body, parting the feathers as you go. Look closely at the skin as you do it under good lighting and see if you notice any grey, black, or red specks moving around, or any clusters of what appear to be tiny eggs near the base of feathers. If you see something resembling that, it could be a mite infestation, especially around the vent. If you see wiggly little straw colored things moving around, those are lice. Let me know what you see. The appearance of droppings and breathing sounds normal?