Swollen air sac in chick

[Shaleen Ann Brown]

Their waterer is not very deep.....and I can't understand the air sac

 

[Shaleen Ann Brown]

Or another possibility is it had an impacted crop, you said there was a swollen bit on its neck. If it was hard than that might be it.

it was bouncy like filled with air

 

[Emma_Bruffy]

it was bouncy like filled with air

Dont take my word for this but I'm about 80% sure it wasnt an air sac, I'm thinking it's the crop. But again I dont fully understand chicken anatomy yet

 

[Emma_Bruffy]

If I had to take a guess I would think maybe it was sour crop. Some symptoms of yours match up to what I'm reading. But I am by no means an expert, I'm just going off of other posts.