bloody vent in year-old isolated rooster

We have a 1 year old rooster who is isolated (has been for 3-4 weeks) in order to let his hen-pecked feathers grow out.  He is a polish chicken.  This morning when I let him out in his run, I noticed blood on the roost.  His vent is bloody as well.  It looks like some blood is dripping from his vent.  I picked him up and he was much more docile than usual.  He looks ok otherwise.  He does have a new sound, sort of a plup plup plup plup in addition to his regular crowing and other noises that he routinely makes. 

Any ideas? 

He did not appear to have any injury in or near his vent, so I don't think he is pecking himself.  I don't think he eats very well since I moved him (he had been with the flock since he was a chick) but since I don't know how much a rooster eats, it's been hard to tell.  The blood was dripping and very red, indicating some sort of wound, but could it be internal?
 

I cleaned him up when I got some help, and the blood is actually coming out of his vent, like he is bleeding internally.  I cut off the poop/blood feathers and cleaned him up.  He is mildly less active but not noticeably.

I've brought him inside to another animal cage where I can keep a better eye on him.  As I mentioned, he has been isolated for some time now.

Any ideas what could cause blood to seep from his vent?

Can anyone help?  He continues to bleed, no apparent injury, just slow dripping of fresh red blood out of his vent.
 

It wasn't coccidiosis.  I started feeding him lots of greens and he cleared up right away.  Previously he free ranged with the hens but when we isolated him to let  his feathers grow out, he didn't have access to all the grass.  That's the only explanation I can come up with because he is fine now.  Thanks for asking.