Hi Jimmy4! It may be related to how fast the blood bag was filled and it may be related to the degree of dehydration your body was in. Had you eaten or drank anything before you went? You need to call the people who provided the service (ie The American Red Cross) to let them know and let your primary care doctor know. Whenever your body loses a large amount of fluid at one time it may make as person feel lightheaded or faint. Thats why some people faint afterwards. That's why they don't let you go home right away afterwards. So today you should avoid operating machinery , drink more fluids than you normally do (Non alcoholic ones like water or juice) rest after each activity and avoid making big financial decisions, until a day or two goes by and you feel better. Talk to your PCP and ask if you should be donating at all. If this procedure was done for medical reasons, let the doctor who.ordered it know. If you ever do this again, drink twice as much water the day before and that may help. Blessings to you! If you have PH or PAH, let your treating physician know this happened. Blessings !!