lethargic chick

I have seven chicks ranging from a week to ten days old and all have been active and doing well. they are on medicated chick feed (today I added a hard boiled egg yolk), have clean water with a nipple and a small amount of apple cider vinegar added, heat lamp, etc. They have been active and peeping, pecking, jumping, eating drinking...
i let them outside for a short time yesterday while the weather was nice and watched to make sure nobody ate anything they shouldn't (the two who split a worm are fine).
one of the smallest, however, has become lethargic since this evening and flops around. others step on her and she just gets dragged by their feet...
i picked her up and she chirped briefly with eyes closed and was limp. I dipped her beak in some sugar water and she mother it but didn't drink. she is currently in the brooder breathing slowly with mouth open and eyes closed.
what is happening?
i am very attached to them all and am also worried it may be cocci and the others can get it too

PLEASE HELP

this is my first time raising them and i have researched hours on the best care

i would get her on corid and all the others too, possibly cocci, it hits quick and kills fast- here is dosage, you probably need to drip the med on her beak the first few times to get it in her quick