Help - 2+ week old chick was fine, now standing in corner

This chick had been doing fine as far as we could tell as well as other 9. Noticed her today standing in corner, fairly still and looking down...slightly puffed feather look to her...eyes open but head in somewhat down position.
We've just been checking her...has whitish/off-whitish ? (poop?? like a wild bird's whitish poop..make sense?) kind of hanging on bottom feathers below vent. Vent itself looks okay it seems. I kind of think she has not eaten or drunk anything today.

We had been giving them a bit of electrolyte & pro-biotics in water. I recently stopped electrolytes, but still just a sprinkling of pro-biotics. Today I put just a bit of vinegar in water. (Okay, I'm not sure why I did this, but thought we had put some in their water before..when we had chicks for first time 2 yrs. ago.) I don't know that she drank any of water yet today or not, so what is in water today is likely not an issue.

I want to say her breathing is labored, & daughter says you can hear her...sounds congested. She thinks chick's crop is not empty, but may feel more like it has fluid feel versus a food content feel??
Also, she will NOT lay down....just stands.

In a way, I hate to separate her...but should I take her out of brooder...or just partition her off or...??

Suggestions Please.

 

She passed...may be respiratory. Worried we will lose others or all. Trying to figure what it might have been from Gail Demerrow book.