Help Please!! Hen broke her Brood and Is now Lethargic, Not eating.

My hen went broody 3 days ago and since she has a strong broody streak, I quickly got her set up in the back of the coop with fresh litter and nice accomodations with her own nesting box. I let her out every morning with the rest at sunrise and she comes out and frantically eats, drinks, poos and often dust bathes. She also has access to her own food and water in her brood area if she wants it during the rest of the day. Today was day 3 and I went out and I noticed she was not wanting to come out and thought that was odd. She was still setting. I came back about 1/2 hour later, opened her door and she slowly came out. Instead of the normal "frenzied" eat, drink and poop" routine she normally does, she was slow, slow, slow. She didn't eat at all and just drank some water and stood around with eyes half closed. She was still "puffed up" broody style and making the broody sounds. I put her back in her area after about 20 minutes and she didnt' seem interested in setting back on her eggs. I left her brood area open and left it up to her whether she wanted to set. I came back this evening and eggs were stone cold and she was in teh corner of the run, alone still lethargic. I threw the scratch out to the hens and she was not even interested in eating it while the other hens devoured it. When they went to the roosting bars, she was not able to get up to them. She has no discharge anywhere and I checked her vent and body for mites/lice and none. She seems to want to drink but again, not interested in food. Weak, and lethargic. Any idea what this could be?? She went from normal broody to ill overnight!

Sylvia