HELP please with lethargic silkie chick

[LadiesAndJane]

I have a 4 week old silkie chick that quite suddenly is not doing well. She broods with two others who are doing fine.
They have been fed medicated chick starter from the beginning, vaccinated for Mareks disease. Kept warm, and dry. They have been using a brooder plate, and she is spending most of her time under there. She is not actively eating, though will take egg yolk and nutradrench if I give it to her, which I have been doing, every 2 hours or so.
There is no sign of diarrhea. No runny eyes or nostrils, no cough. No sign of injuries or parasites.
What could the explanation be? She seemed fine yesterday, then today not.
I have not experienced this before with my own chicks at this age.
I have with hatchery chicks in the mail, who failed to recover from the trip.
I hatched this one myself from my own birds and she has been a normal lively chick until today.

 

[HappyDuckie]

Could she have been pecked on the head? I know that silkies and other vested birds can be more prone to injuries that way.

 

[Eggcessive]

Had she been doing well up to now? At her age, I would worry about possible coccidiosis. What do her poops look like? Is there food felt in her crop?

 

[LadiesAndJane]

Could she have been pecked on the head? I know that silkies and other vested birds can be more prone to injuries that way.

No, I do not think so. Thank you for replying!

 

[LadiesAndJane]

@Eggcessive
Thank you for replying. After reading through some threads I decided to go ahead and treat for coccid. I have Corid on hand. Just put in the water now and got her to drink some. Poops look normal. The other two chicks seem to be unaffected.
Her crop feels a bit distended but does not really feel "squishy". Is definitely not hard at all. I have been giving her egg yolk and nutradrench, but she is not eating without encouragement and not eating her crumble at all.
She was fine yesterday. Symptoms started this morning.

 

[Eggcessive]

If you have the liquid corid, you can give a boost of a couple of drops of undiluted Corid twice a day in addition otmthe Corid water.

 

[LadiesAndJane]

If you have the liquid corid, you can give a boost of a couple of drops of undiluted Corid twice a day in addition otmthe Corid water.

I have the powder. Can I give her a little bit more concentrated? I figured out how much to put in a quart and all 3 chicks are drinking out of it. She only does if I dip her beak in. She will drink the eggyolk and nutradrench off a tilted spoon.
She is the daughter of my white silkie pair that I just sold, so she is very precious to me.

 

[Eggcessive]

You can mix 1.5 tsp of of the powder in 10mlor 2 tsp of water and that is equivalent to the liquid undiluted Corid.

 

[LadiesAndJane]

Thank you!

 

[Wyorp Rock]

They have been using a brooder plate, and she is spending most of her time under there. She is not actively eating, though will take egg yolk and nutradrench if I give it to her, which I have been doing, every 2 hours or so.

Good advice from @Eggcessive
Just to add - omit the Nutri-Drench during the course of treatment with Corid.