sick chick?

We took our chicks outside to run for a couple of hours yesterday since the weather was so nice and warm and they had a grand old time digging up bugs.  They even found a few worms that they played tug of war with:)  They were very excited to be out of their box and in their big space, and they were fine later that night when I put them back in their cozy box as well.  This morning I gave them fresh food and water and left them in their nice warm box with their light and left.  When I got back they were peeping and running around the box but one was in the corner by itself and just sitting like it was cold even though it was directly under the light.  I took the rest out to "play" in the sunshine and left this one in the box to stay under the light.  It is looking around but just sitting there and if I pick it up it will close its eyes and not move or peep at me.  Now I am worried it might have gotten ahold of a bad bug or weed:(  Or if it is sick that it might infect the rest of them.  I was thinking of letting it stay in the box with the light and moving the rest into a different box with another light to keep them away from it but if it was sick they would already be exposed, right?

oh, and they are a little over 3 weeks old.  There is nothing wrong with it that is obvious, no poop on the butt, no crusty stuff on it's face, doesn't feel like it is cold but it is just acting like it. 

well I ended up getting it to drink a little sugar water with a dropper.  Then when it finally opened it's eyes and raised it's head I gave it some regular water.  I have been in here watching over it and it just raised up on it's feet and is taking a few steps, a little drunk looking but at least it is walking.  I will update again later.

well I am sad to say I just went and checked on it and found it dead :(  It was so weird, I don't know if the sugar water hyped it up one last time or what.  The last time I checked on it, about an hour ago, it was actually walking around better, and it was drinking from the waterer on it's own.  I just went to check one last time before I went to sleep and it was on it's side and it was gone.  I have no idea what happened and now I am worried about the rest of them:(  I really hope the kids don't notice any time soon.  They have named all 8 of them but I don't think they can really tell which is which except for the oddball EE.