What Causes Recurring Fainting Spells?

im having very bad fainting spells. the most recent was today 01/7/2014. ive had them before when I was younger. when I went to the doctors the last time all my tests came back okay. there wasn't anything wrong. the are infrequent, sometimes 2 months apart other times 2 years. I just don't understand what is happening to my body.