I tried to donate blood a few days ago. I am a healthy young woman, fit and I eat healthily... Most of the time. I am not low in Iron (they tested it at the blood donation centre). What happened was, they started to take my blood. It was all going well for a few minutes. Then all I remember is lying on my back, covered in sweat with 2 nurses standing over me, trying to bring me back to my senses. It was the first time I had tried to donate, so I am wondering what the possible causes of my fainting could be. I have normal blood pressure, and I don't suffer from any diseases, so why did I faint? And what can I do to prevent it from happening next time?
Any help would be much appreciated.

It could have been a low blood sugar moment, sweating is a sign of that.. Understandable if you had not eaten breakfast, were lacking a meal before you went to give blood, then could have been low blood sugar, or possibly just thought of losing blood could have done it. Was this the first time you had given blood. They usually give orange juice when finished giving blood because of the low blood sugar reason.

Well I was actually sitting down while they were taking my blood, but when I woke up they had inclined the chair so that I was lying down. I had eaten breakfast, morning tea, and lunch, so it wasn't lack of food either. I'm not a squeamish person, so the thought of giving blood wasn't disturbing either. It happened while I was donating, not after. I had not attempted to stand up, as I never finished donating. They normally take about 650mL od blood, but I fainted before they had even taken 350mL. And yes, they did give me orange juice.

Nerves or worry from giving blood ? Have you went back to your family doctor for a blood work and exam ?