HELP!! Hen with droopy comb, lethargy, yellow poop?

Hello all,
My hen has been getting more and more sick over the past few days and I'm very concerned about her. She's been acting extremely lethargic, keeping her eyes shut most of the time during the day and just sitting in one spot whereas she used to be roam around the yard a lot. She's also had some concerning looking poop- it's yellow and white and runny. Her comb has become droopy, her feathers seem to be greasier/dirtier than when she was healthy and the only thing she's been eating is very small amounts of cracked corn and grubs that our rooster brings to her. She's had a lot of poo buildup on her for a while now and I'm worried she may have some sort of infection from it, but that's just a guess from me. Can anyone please help diagnose her? I've never had a sick bird before and I'm very worried!