Blood in the stool does not occur with every strain of Coccidia. It doesn't even occur often. Most cases of Coccidiosis will be fatal before blood is ever seen. If you wait to see blood, you will have dead chickens. There doesn't have to be a change in stool either. Again, your chickens...
At 15 weeks, I wouldn't have expected an overload of worms. I would have expected Coccidiosis and that would have been how I would have treated. Corid in her water for 5 - 7 days.