I actually had a collapsed lung a couple of years ago so hopefully I can be of some help.
My lung collapsed spontaneously while I was watching TV. I got a pretty sharp pain on the left side of my chest for about 30 minutes and deep breaths were painful. I hopped on the internet and sure enough, tall thin males are actually somewhat prone to spontaneous lung collapses and all my symptoms fit the bill. I decided to go to a physician just in case and, not surprisingly, he tried to diagnose me with something else (walking pneumonia I think) and said I shouldn't trust the internet. He also told me not to run for 4-6 weeks (yeah, right).
I believe this whole ordeal started on a Wednesday. I took the rest of the week off and started running the next week, putting in a 109 miles. The first mile was typically excruciating and then the pain settled for some reason. After 2 weeks I was running fine. A CT scan during all this revealed I was right and my lung had collapsed. A pulmonologist told me I couldn't run until I was 100% re-inflated and I would have to get a CT scan every 2 weeks to check progress. I continued to run anyways, and PR'd in the 5k and 10k