I suggest you get a sample to the vet asap. And maybe improve your coop hygiene (no rudeness intended).
Quote: A fecal sample taken to your vet for a fecal float is a good idea.
The picture would be great.
There are several strain of Cocci, Corid will generally treat all strains, while Sulmet will only treat two.
Coccidia are almost universally present in poultry raising and in the soil. Cocci will always be present no matter how much cleaning you do, where it becomes deadly is when there is an overload. Generally this happen in chicks, but can happen in adults as well especially when exposed to new ground or environment, (there may be a strain they haven't built immunity/resistance to). Overtime chickens build a resistance to what they are exposed to, but Corid/Amprol blocks Thiamine uptake which the Cocci feeds off of, essentially slowing growth so the chicken can build that immunity.
This gives a good overview of what Cocci is, how it's found in the environment and treatment.
http://www.easychickenry.com/coccidiosis-in-chickens.html
Explanation of the different strains of Cocci.
http://www.merckvetmanual.com/mvm/poultry/coccidiosis/overview_of_coccidiosis_in_poultry.html