If you could please answer the questions in the second sticky of this forum, but answer the questions in this thread, it'll help us have a more clear picture of her history and the flock history. That will enable us to help you more accurately.
There are so many things that cause lethargy in chickens.
First thing I would recommend is to do a thorough checkover of her at night. Night time because mites sometimes don't feed until the night and you're apt to only catch them then except in very bad infestations.
Check several things thoroughly and please report your findings to us:
Parasites: Check her body carefully - all feathers, fluff, skin, feather shafts, etc for parasites. Ruffle through her feathers over a white sheet.
Weight: Check her keel, breast bone, and how much meat is on each side of it. Is she thin there?
Abdomen: gently palpate her abdomen between her legs and back towards her vent. Very very delicately. Do you feel excessive mushiness or lumpiness?
And by the way, foggy means that the heat feels about 15 degrees more hot than it is - that's how humidity works, so don't discount it yet. BUt let's rule out other things.
Have you been worming her?