Fainting followed by vomiting. For this?

I faint after long standing or long sitting (when I get up).. my symptoms are like.. I start yawning multiple times (I get a clue that am about to faint),my hands and feet starts feeling cold, I get very conscious of myself (everything around goes out of focus) when I feel that am about to fall I carefully sit down immediately and faint. My eyes dark out and after a while I get to see the surrounding.. I vomit immediately and after vomiting I feel little better. This happens once in a while.. The last time I experienced this when I stood for 'few mins' in a crowded bus.. I felt suffocated and everything else followed the same way. I was diagnosed with a mild spina bifida during primary school. What could be the reason for this?