My 1.5 month old flock of Speckled Sussex were recently let out into the run of their coop and today I noticed one that appeared to be gagging and was cranning her neck around a lot. When not stretching her neck off to the side and back a bit her preferred position is to sit there all scrunched up with her head close to her body. The feathers on her head and neck have the appearance of a chicken that's molting. Her chest also looks puffy in comparison to the others. She seems lethargic and disengaged while the others are quite active and scratching around her. I went in to pick her up and try to examine her and she barely moved and her chirping seems muffled or just very quiet. I also noticed a bit of drool come out the side of her mouth when I was holding her. This appears relatively recent, like within the last couple days. They always have access to fresh water and feed. A friend of mine suggested it could be lice, electrolytes, or coccidia. Any help would be greatly appreciated. I want to help her feel better!