Spontaneous pneumothorax, sudden collapse of my left lung from...nothing... I sat up from the bed to throw something away and felt like I got stabbed in the back.

Felt very strange, I laid down for an hour or so and felt okay, tried to go to the bathroom and couldn't get out a full sentence without needing a breath.

Girlfriend (now wife) brought me to the ER, had surgery to fix it. In my case they had to cut the lung such that it healed and stuck to the ribcage, preventing it from collapsing again.

The feeling of your lung being inflated is wild.