Air like bump on chicks neck

[sierraparker1]

This Chick is 4 days old. She has been smaller than the other chicks since day one. Just this morning she was running around with the others acting normal. When I checked after work, I saw she was acting lethargic. I saw she had some paste butt and I quickly cleaned her up. After cleaning her up, I noticed that her neck was swollen and looked like it was full of air or something. I touched it and it did not feel hard. She’s still not moving much at all. I did see her stand up and poop. It was runny and a pale yellow color. Anyone have any idea what is going on or how to help her?

 

[sierraparker1]

Here are pictures of the chick