I'm Seeing Things

Little particles of light flitting before my eyes and on walls and on computer screens and books and just everywhere. And I don't mean the kind of imprints you see when you look into bright light for too long and then look into a dark area or vice versa. I see these little shiny specks just poppin' up out of anywhere at any hour of the day in any light all over the place.
I've never tried to touch one (can't give anybody a reason to call me crazy), and I don't know if I'm the only one seeing them (I strongly suspect so, because this chick at work the other day just walked right through a coupl'a them, and never even flinched).

I used to see this kinda thing when I was younger - everywhere, but they went away. Now they're back again, and I'm wondering what they really are. What causes your eye/mind to suddenly play tricks like that... I'm not crazy, and I'm not sick either. I'm just seeing things that nobody else does... whatever that makes me, I'm not crazy!