I've had a similar problem with a few chicks my advice is to isolate her from the other chicks for the time being. The exact same symptoms were exhibited by the 3 of my chicks a few years ago. I believe they had some sort of disease, intially they ate fine like her, but they also were peeping...
What do you feed them? sometimes tablescraps can cause this especially if they eat to much fruit. Another possiblity is coccidiosis, is the diarrhea bloody? The water they drink could also be having this effect or they could possibly have a vitamin deficiency. Have you given them any...
not to discourage you but I had a chicken with a very similar problem, her wing was hanging and her eye was closed on one side intially, eventually in about three days her toungue started hanging out and she couldn't swallowed, based on her signs and sympotoms I believe it was botulism. She died...
My chicken is about 4 years old she's a barred Plymouth rock and in generally good health. The day before yesterday she swallowed a pieces of glass, I slit her crop open and removed all the pieces of glass or so I thought. Right now she's fine but I can still feel a few pieces of glass in her...