If you could tell us hold old she is? Has she been laying? If you could post a picture of the poop it helps.
Sorry this is happening and I can’t give you answers. Others here can though but I think they need more details.
I definitely would keep her separated from the other flock. Put her is...
Is there any blood in her poop?
I hope an expert can tell you. I know pictures can help if you can post a picture.
Different things can cause milky white poop. I had a hen that had white diarrhea and she had an infection of her reproductive organs. I had to give her antibiotics. I had Tylan on...