I recently ran across this image on a Facebook page and was honestly appalled that a place where people are encouraged to visit for information and discussion on a variety of paranormal topics would post such a fear-mongering piece. If you have ever felt dizzy or felt as if you were going to pass out upon standing, before you call a priest, please call your doctor!
What you're experiencing is a medical condition known as orthostatic hypotension, and while its fairly common and usually benign, it some cases it can indicate a more serious health issue. Basically what happens is that blood pools in the lower extremities during a change in body position. As you quickly rise up from a reclining or sitting position, your blood pressure quickly drops. Certain medications, including anti-depressants, can increase the condition. Almost everyone will experience this, at least a couple of times, and if it doesn't happen with too much regularity, its perfectly normal and not a medical emergency.
However, if this is happening frequently, or if the bouts of dizziness last longer than a few minutes, you need to contact your doctor, as it could be an early warning sign of different neurological disorders, diabetes, etc.
The Mayo Clinic has done a wonderful job outlining the symptoms, causes, and treatment options for this condition. Please go check out their site for more information!