Unfortunately chickens aren't the most hardy animal. If they've got something and you don't have the meds already, they get worse very quickly. I'm sorry. Just try to make her comfortable.
Well, it doesn't sound good whatever it is. Someone's gotta chime in here soon. Did you say they were free ranging? Did they eat a lot of vegetables or fruits? When my chickies eat lots of fruit their poo is really liquidy. Can you see if there are worms in the poo?