When you kneel down for a while and suddenly stand up, there is a drop in blood pressure. Normally, in someone under 60, the heart picks up its rate quicly and blood vessels squeeze tighter to compensate, to quickly increase blood flow to the brain. Everyone sometimes feels dizzy when they suddenly stand up, even my 15 year old son. Dehydration is a common factor which can make this worse, when there is a lack of fluid in the circulatory system. If it happens occassionaly and not every single time, then it's normal im afraid. The slower you get up, the more time the body has time to adapt, but it should literally take a few seconds.