Strange bulge on a chick

[chibifarm]

When I bought this chick a couple of weeks ago, I later noticed it had a little bald spot on his lower left side of his neck/upper left side of chest. It has since puffed up to this bulge. It's not hard. It feels like its a bubble. Anyone know what may be the cause and how to treat it?

 

[GitaBooks]

I think that is where they give chicks their vaccinations, and it could be a reaction from that. I've had a chick that wasn't vaccinated that also had air bubbles/water bubbles under its skin (this one ended up dying, however).
Could it just be the crop? (that is on the right side, though, just thought I'd check).

Best of luck with your chicks!