Why am I so afraid of this?

I'm very afraid of needles and I have no idea why. I have 8 piercings,6 in my ears,1 in my tongue and 1 in my lip. That's not my problem, it's only blood-work. I get so scared, I start to shake and my hands get cold and clammy. It's embarring because sometimes I scream while it's getting done and the security gets called in because I screamed. I know of some people that have to get held down, I don't, but I can't understand why I'm afraid of something so petty. Please give me at least an idea.