Im not sure if you can actually tell who is pooping it by the size of the poop. But Cocci usually comes in growing chickens (whilst they grow they develop an immunity system for it). You should try and catch them pooping so you know who it is exactly. If you look at their behinds you might be...
Maybe it is the move, I know it happens. It happened to my 6 week old chicks but left after 5 days. You should move them until you know what it is. You should clean out all the bloody stools. You can also give them yogurt.
How old are they?
It is hard to tell exactly but I think it might be Cocci. Sometimes the chicken can also have worms, or another kind of parasitic disease. But in other cases it can just leave, I had a chicken with red watery poop a week later it left. But did you just move the chicken? Or added a new friend...
Hello, if the whole poop is black that doesn't matter, it is completely normal. But usually the chicken has Cocci if the whole thing is red. Can you post a picture for me?