I don't know where you are - your profile avatar doesn't say, but excessive heat can negatively affect chickens just like it does to humans. In extreme heat cases, we have brought birds in the house to cool down during the day if necessary. Fans in a run/coop help knock the heat down; so do...
+1 on the comments above. Look her over for sure, and check for external parasites. You didn't mention worming or medications, but I sense that's not the issue. Instead, I don't think she's getting the right thing to eat either. She's definitely not getting enough protein, and even the...
If you can, post up some photos of the bird, and provide some particulars like the following:
How old is she? 3 years
What does her diet consist of?
When was the last time you saw her eat/drink on her own?
Have you checked her crop (full at dusk, empty by morning)?
Does she free-range?
Do you...