It can be explained by light-headedness whereby you feel dizzy for a few minutes. It can be caused by various factors like getting up too quickly after lying down or sitting for a long time. It can also be due to a sudden drop in blood pressure or dehydration too. You can get more information on this at, http://www.mayoclinic.com/health/dizziness/DS00435/DSECTION=causes.
it is something about your blood pressure when you stand up. This happens to my husband a lot he will black out and it has caused him to pass out. it happens to everyone just some are worse than others and it really depends on what you have eaten drank and the activities you have done in that day.