Blood/intestinal lining in poo.

Hi. I found some red matter in my chickens poo this morning and immediately thought of coccidiosis. However after looking at some photos of chicken poo (how varied!) I'm thinking it might be intestinal lining which apparently is no cause for alarm. My question therefore is how long does this last and at what stage should I be thinking it is something more sinister? Any advice would be greatly appreciated thank you 

I saw them a lot when I raised some table birds but when I changed their feed to a more expensive brand the all but disappeared.