Sometimes, particularly when around crowds of people, I get a feeling that I'm not actually there. As if I'm some kind of ghost who is walking about among the other people observing them. Also, it is as if I have nothing in common with the people I see around me. It is as if I have been put among them, but actually belong somewhere else.
Ever seen the commercials for Zoloft? The little bubble dude that walks around? You're describing one of their commercials.
At the minimum, you've got social phobia. At the maximum, schizophrenia or dissociative disorder.
I'm not telling you to go take some Zoloft - but I'd suggest you go see a counselor and find out if you do need either some therapy or some form of medication. Most people with social anxiety can just get over it by learning techniques for breathing and thinking nice thoughts about yourself, that type of thing - people with something more major need the meds.
(Psychology Major in college - dropped out 3rd year - I've had my own psychological and "invisibility" issues - trust me on this one.)