Sick chick - is this cocci?

I posted this in emergencies, but posting here in case i can get help:

We ordered five bantam chicks from Cackle. Received on Friday and all were doing great. On Sunday AM, we noticed one was lethargic and spending time alone not eating or drinking. I separated her and have been administering electrolytes and slurry egg yolk with limited success. She is listless but won't lay down, just eyes closed on her feet. I thought for sure she would die in the night but she's still here. Her poops are normal considering I've been putting water in her. Not bloody.

Could this be cocci?? This is my first batch of chicks so I don't know. They are on medicated feed but I read it does not always work. I did get some sulfadimethoxene with the chicks; should I use it on the other four as precaution? They seem to be fine. And is it too late to use on the sick one?

The four others are on pine shavings with a few paper towels changed every few hours. Trying to stay scrupulously clean over here....

advice is appreciated.