Quick death

My runt of the pack (9-weeks old) passed away last night, but I'm not sure why. The day before, I noticed she was in the front yard by herself looking for food. I assumed the group left her while she was busy eating and she just didn't notice until they were out of site, so I had her follow me back to the pack. Well, yesterday day when I got home from work, I noticed she was sitting under their favorite tree by herself and it looked like she was sleeping. Nothing out of the ordinary since they do take naps under it, but the rest of the group was on the side of the house scratching for food. I went to check on her, and she opened her eyes and took a few steps and started to fall asleep again. I picked her up with no hassle, which she usually runs from us and she just sat on my lap while I tried to feed her treats. She wasn't having it, so I placed her back on the ground.

When I returned later that night, she moved in the yard with the rest of the group, but she was sitting in the grass sleeping again. I took her inside with me for the night. Set up the heat lamp, fresh food and water, but she just say their. Looked to see if she had a hard crop, but it was empty, so I knew she disnt eat all day. No visible parasites present. Was breathing normal last night, no wounds, puffiness, ect.

The only thing I can think of is possibly coccidiosis?

The rest of the hens look fine this morning. Jumped right out of the coop and started scratching.

Anyone have any thoughts?

Coccidiosis would be the first thing I would think of in a chick under 4 months old. Symptoms are sleepiness, weakness, hunched or puffed up posture, not eating, and runny droppings with yellow mucus or blood. Corid powder or liquid from a feed store is what you need to use for treatment. If you still can get to the body, and open up the abdomen, you can look at this second link for what intestines look like with cocci. Here is a good link(s) to read:
http://www.the-chicken-chick.com/2012/12/coccidiosis-what-backyard-chicken.html
http://www.thepoultrysite.com/publications/6/diseases-of-poultry/206/coccidiosis/