Neurocardiogenic Syncope

Neurocardiogenic Syncope is where a person's blood pressure rapidly falls, and blood flow to the brain becomes very low. The person loses consciousness, usually for only a few seconds. Neurocardiogenic syncope can happen suddenly, in response to a startling event or strain (a needle stick, pain, fear, cough, or defecation). It can also occur after prolonged standing, heat exposure, or exertion, and sometimes stress can cause it too. Some people experience a period of ill-feeling for a few minutes before actually passing out. After a fainting episode, further testing is often recommended to make sure a serious cause of syncope (such as an abnormal heart rhythm) isn’t responsible.
More info: http://answers.webmd.com/answers/1198623/what-is-neurocardiogenic-syncope