Why do I pass out after seeing blood?

Okay well yesterday I was at the doctors office with my girlfriend because her toenail got almost completely ripped off. I was fine when the doctor removed the nail but when she started working on it more I started to feel queezy, the next thing I knew was that I was laying face down on the tile floor. The same thing happened when I had an ingrown toenail remove a couple years back but those were the only times I have ever fainted. I am a frequent hunter and I play football so I am used to seeing blood but I dont know why I fainted from the toe operations, can someone tell me why am how I can overcome this? Thanks to anyone who can help.