Really sucks for Flash and the Avs. Very productive since the trade and couldn't come at a worse time for the team given the closeness of the playoff race in the west.
Ahhh, may have found the source. DVT (Deep Vein Thrombosis) can cause this. The clot forms (most likely in the calf region) and breaks off, traveling to the lungs where it gets clogged in a capillary. Obviously this leads to issues with chest pain, lack of O2 to the lung, etc. Basically it's like a stroke, but instead of going to the brain it goes to the lung. Kind of odd to develop DVT at his age though without having some other factor (more common in those who are hospitalized or sitting for long periods of time, coming off of surgery, pregency, that whole spiel). He's very lucky however, this can be suddenly fatal if not caught quickly.