Young chick with bloody poo and gray swollen butt

[harleyjo]

I went out to my brooder this morning to check on my approx. 3 week old chicks and notices a bloody poo on top of the waterer. So I did a bottom check and one of the small chicks had a gray, swollen bottom and a little loose bloody poo running down it.

So I moved it to a different brooder by himself. He is active but I don't know what is going on. They are on medicated chick starter. These are Delaware chicks.

Any ideas, all the other chicks look fine.