So sorry but just remember you didn't kill your chicken. They are suppose to be grazers and foragers and sometimes freak stuff happens. I Keep mine on pasture and never had this happen so Not Your Fault.
Also, someone mentioned in the beginning to give fresh orange juice. I don't want to put anyone down but you shouldn't feed citrus to birds, especially anything acidic when they have watery poo. I don't feed citrus to my birds ever. Have been told by my vet that it's not good for them.