Sick Chicken not eating.

Hello. I'd like to start out by saying I'm new to owning a flock of chickens. I have a hen that's not drinking her normal amount of water and barely eating. It started about two weeks ago. I have kept her away from the rest of my flock. She lays in the same area all day. Her eyes seem alert but no movement and her food and water intake are VERY minimal. When it first started I thought she was egg bound but she layed an egg that same day and the day after. Her crop does not look the same, it's not perky like it used to be and it's pink in color. I have been giving her electrolytes and a multi vitamin. She does not poop unless I stand her up and when she does go it's usually very watery at first and then it seems normal but in a large amount. She also has a lot of small droppings right after the initial large dropping. Today her poop was more like a yellow/white paste and then she proceeded to drop one that seemed normal but extremely smelly. Her vent seems normal, no droppings stuck to her vent or her feathers. She is very thin and I could feel her breast bone and the bone right below her neck. Any thoughts and what could be wrong with her? I'll post pics if needed.