I have never used Corid, only sulfadimethoxine (Sulfamed G) to treat coccidiosis, but I would be surprised if a drug designed to treat the disease would cause identical symptoms to the disease, to the point of causing death.
I would be more inclined to believe that the other birds in your flock were also suffering from coccidiosis, but only started to show symptoms after you started the treatment.
If it was me, I would keep going with the Corid for the full 7 days and see how things go. If there's no improvement then maybe you should contact your local vet and ask if it's possible to go straight on to Sulfamed G. I don't know if it's ok to continue one treatment directly after another, or if you need to leave a break between the two.
From what I have read and understood online, Corid seems to be a 'gentler' treatment for cocci, which can be used long-term at a preventative dosage, whilst Sulfadimethoxine is an antibiotic and therefore more agressive. (I may be wrong about that, and am happy to be corrected if necessary!)
All the best for you and your flock - let us know how things develop.
