Why do I always get the feeling of not being "here"?

Almost always, especially in public situations, I feel as though I’m in a constant dream-like state. Its actually quite scary sometimes; I could be doing something such as walking through the mall, and then I’ll realize, “Woah, I’m actually here, this is actually happening and I’m not dreaming”. And that sudden epiphany is accompanied by a lightheaded feeling that surpasses after 10 seconds. Does anyone else ever feel like this? What could be the reason for this?
(And I don’t do drugs, ever, so trust me, this is no result of drugs, however crazy I may sound.)