It seems when you treat chicken for mites the powder gets in there vent and drys it up,At least mine did that.
Check for mites again as @Wyorp Rock said.
Sometimes you may need to use a Poultry Dust or Permethrin spray to get rid of a lice/mite infestation.
DE can be helpful when used as a preventative in dust bathing areas, sprinkled in nesting boxes, etc., but an infestation may need further treatment.
When you mention that she keeps putting her head toward her back/rear - is what you are seeing a preening action? Chickens have an oil gland toward the base of their tail, they will wick oil onto their beak and spread it throughout their feathers (preening). Is she acting similar to this? -- https://www.youtube.com/watch?v=-aMudfkzLa4 This is a good example of a hen reaching her oil gland and then preening her feathers.
Let us know how she is doing?
Nutrition is her issue....Chickens have very specific dietary needs....Like all Birds do......Pop corn is like feeding your Kid candy all his life..........Age appropriate feed, water granite grit and oyster shell to layers is all they need......Many Chicken issues can be prevented by feeding the right diet.....
Stop giving it to her.........