When I rescued a chicken that fell off one of the trucks here (I called her Lucy) She had runny poo for a while too. I asked around and internet searched. Seems that they feed them something that will make them want to drink more to build up fluid in the body. Adds weight and makes the meat retain water so that the end product is moist.
Does yours have baby blue eyes? Lucy did until she got a little older. When she started to lay she ran into big problems, (prolapse twice) and at 6 months she had heart failure. But at least she had that many more months of good living, unlike the first 45 days of her life.