I've been feeling lightheaded whenever I get up from a sitting postition for around a year now. But today when i got up and I went to the bathroom I got really lightheaded and then all of the sudden I wake up and I'm lying on the ground... It was really scary since I've never fainted before and I don't know why this happened. So I went back to my couch because I was really scared and then I got up a while later and it happened again! Fortunately it didn't happen again throughout the day but I'm really scared it might happen in school or something.... If it helps, I'm 15 and i weigh 118 pounds, and I'm around 5'7. Why did I faint? What can I do so it won't happen again?