Sulmet is an antibiotic as well as a coccidiastat. I think you need to call your state vet and see what is involved in getting a necropsy done. Whatever it is is moving through your flock pretty rapidly. You can also take some of the poop samples (preferably mixed together, not separate, and ask any vet to do a fecal float. That will rapidly tell you if it is worms or coccidia (make sure they know you want both looked for).