So sorry to hear about your snail(
I don't know much about them, I guess that's the lung? But I've seen this in people and dogs. A bubble will appear on the the neck or where ever, looks like a big water filled bubble. It's like a cyst on the outside instead of the inside. Don't know if that's the case here, but this is the only time I've seen bubble like growths.
The first picture was taken just before I lanced the bubble. The second pic was a few hours later (you can see his flesh looks lighter, more yellow.) The last picture is the snail today, resting on the bottom of his tank. I picked him up just a few minutes ago and he snapped himself shut -- tight. So far, there's improvement. Yesterday he barely reacted to being handled.