My 18 month old Buff Orpington hen has developed a reddish brown diarrhea in the past week or so. She is eating and drinking well and appears to be otherwise healthy. She lives in a house and run with 5 other of the same breed and she is the only one with diarrhea. I have put buttermilk in the waterers for the past 3 days and it has tightened up the feces just a little bit. What could this be??