Many different conditions can cause fainting. These include heart problems such as irregular heart beats, seizures, panic or anxiety attacks, low blood sugar (hypoglycemia), anemia (a deficiency in healthy oxygen carrying cells), and problems with how the nervous system (the body's system of nerves) regulates blood pressure. Some types of fainting seem to run in families. I have fainted from dehydration.
A simple episode, also called a vasovagal attack or neurally-mediated syncope, is the most common type of fainting spell. It is most common in children and young adults. A vasovagal attack happens because blood pressure drops, reducing circulation to the brain and causing loss of consciousness. Typically an attack occurs while standing and is frequently preceded by a sensation of warmth, nausea, lightheadedness and visual "grayout."
If you have fainted you should call your Dr. for his input.