Honestly, in a case this bad I'd just treat her with chemical stuff first. Mites can take a bird down fast and kill them. These are blood taking mites, apparently. You can use sevin in the bedding and on her, or adams flea mist from the petstore. You'll want to treat and then retreat to kill in 10 days. THEN after that, start using DE. But you only use a little. there have been a lot of cases lately of people using it profusely and having birds with respiratory problems as a result.
Personally, if I see this many mites and it makes a bird anemic like yours is, I use drop-on ivermectin in combination with the sevin in the bedding, then the DE as prevention.
As for what causes it, probably nesting birds brought the mites in.
In the mean time, please make sure she gets super nutrition in. Boiled egg yolks fed as a treat, and pedialyte or organic apple cider vinegar in the water (1 teaspoon per 1/2 gallon) will help.