This has been happening just recently, within the past month. If I twist a certain way while lying down on my left side, I'll feel a sharp stabbing sensation in the left side of my chest. Whatever it is, this motion is what triggers it.
After that first sharp stabbing sensation, assuming I do not move and take shallow breaths, I will not feel anything more. However, as soon as I bend, twist or breathe in or out deeply, the pain will come. It's intensity depends on how much I move. If I move very little, it won't hurt much. If I move a lot, the pain increases. Eventually, usually after a half hour to an hour, the pain will dissappear on its own.
I am typing this now after having woken up from yet another experience. This time something peculiar happened; I felt like yawning, but actually acted on the impulse this time. Rather than percieving a sharp increase in pain like I expected, I felt a bump or pop in the left side of my chest, and the pain vanished immediately.
There's still a little bit of... not pain really - it feels kinda like heartburn, except very mild, and on my left side where I was experiencing the sharp pain before, but it's otherwise gone now.