Why does my skin sparkle in the sun?

Ok, so before you call me a vampire, I get it enough from my friend at school, lol. It's sparkled LONG before twilight came out. It depends on the amount of sunlight that hits my skin to be able to see it. It's Magenta and Yellow sparkles, and yeah. No I don't wear make-up/lotion/sunscreen. Plussss I never get sunburned soooo maybe that helps? But yeshhhh. Thnx ^_^