Chickens Bleeding from Rectum

Hi ,
I am a first time user of this forum so I am hoping this is the proper place in the Forum to list this.
I have 6 young Roosters about 4 months old. They might be a tad bit older. I noticed blood in their pen this morning and thought a fight took place. I ruled that out when I saw no signs of facial injuries or to any of their bodies, so I stood and watched them all for a bit until I saw one of them spew blood from their rectum. I immediately removed the bird. I went down and checked on the others several hours later and noticed more fresh blood which tells me there is another one with the same problem although I don't know yet which one.
I've had these chickens for roughly 6 weeks. Started them on Chickie Medicated Food not knowing what they were feed when I bought them. Then promoted them to Grower Food. For several weeks now they have been on layer ration same as my hen's are.
I separated them from the hens around 4-5 days ago. Fresh pine shaving is their bedding, fresh water daily, oats and grit.
I'm at a lose as to what is going on? Any suggestions?
Thanks so much !!

You're sure they don't have wounds? Have you picked them up and looked carefully? (for those interested, I have a video that shows how to pick up a hen and examine the vent area here: http://hencam.com/faq/handling-and-picking-up-your-hens/
A bit old for coccidia, but blood in the manure is a sign. I've never bothered with grower ration (but then I keep hens not roos) and keep the youngsters on chick feed until laying time. Do you still have the medicated feed? I'd put them back on it.

I put them back on the chickie medicated feed today. They also have never been outside. We just built a new chicken house and all my chickens due to various ages and sizes are all housed separately in individual pens. I have checked all the other pens and none of the hens or babies are displaying those symptoms. They all seem to be eating fine as well as drinking.

OMG. You need to get them on a med for coccidiosis. Like Corrid or sulfadimethoxine. You might try a feed store. Or Tractor Supply

They don't all get it, but I treat everyone. The cocci is in the dirt and in chickens, but sometimes it gets overwhelming to a young chicken.
I've had chickens 4 months old get it. It depends on the exposure. Usually by that age they have resistance to it. But not always.

I have no idea what Cocci is. The worst of it is the farm store is now closed so I won't be able to pick up any medication until tomorrow. What exactly is it ?
I started raising chickens a year ago August so this is all new to me.
Thanks so much for all the feedback and the video was very enlightening

Younger chickens can sometimes get overwhelmed with cocci from the ground. They also carry a few cocci around. The cocci, depending on the strain, attacks certain parts of the intestines. It's very common.

Hi All
I appreciated everyone feedback and thank you so much ! I consulted with 2 of my local Vet's . Would you believe no one in our area much less the Vet's had any medicine to treat coccidiosis. My husband and I drove an hour to the nearest city and even then, none of the farm stores or Vet's had the med's to treat my chickens. Finally we thought of Champions Feed Store and they had something there that was formulated for coccidiosis. Out of approx 60 birds, I only lost 3. Thanks again for everyone's prompt reply back to me.