Passed Out!!

Ok so how does this happen? I was at the Dr.'s office today and things are good so far. I need to wait on some results to come back but nothing should be too bad. However, I just had my blood drawn. I watched, yea it hurt a bit, and then she put a band aid on it. The next thing I know I had 4 nurses and my Dr. in front of me trying to wake me up. I had PASSED OUT! This hadn't happened to me in a long time, like 18 years. I am NOT afraid of needles! What the heck is wrong? Oh well I hope it doesn't happen again.