Why do i have a feeling of nostalgia before i pass out?

Underlinig problem: Your sense of nostalgia is being triggered by some sort of underlining condition. In your case passing out might be sort of defense reaction of your body against that feeling. This type of defense is not normal however. You definitely need to be seen by doctor and evaluated thoroughly. Start with your primary and follow his / her advise.

First of all, see : Your dr and find out the cause of your passing out - it could be from a heart condition, brain pathology, seizures, anxiety , panic attack etc. The nostalgia is your brain's neurochemical response prior to your passing out. Take care!