Transient loss of consciousness after penetrative sex may represent an episode of vasovagal syncope. Vasovagal syncope is a common type of fainting triggered by psychological or physical stress, such as seeing blood, straining during a bowel movement, or intense sexual activity (deep penetration hitting the cervix, which has lots of nerve endings), which causes your heart rate and blood pressure to briefly drop (you can confirm this with an automatic blood pressure cuff available online or at your local pharmacy). In most cases of vasovagal syncope, you have some warning that you are near fainting. These signs include "tunnel-like" vision, nausea, dizziness, pale skin, feeling hot or cold, and/or sweating. A vasovagal episode usually is not anything to worry about, as long as your doctors have ruled out other more serious causes. If you experience a vasovagal episode, lie on your back and elevate your legs with pillows to increase blood flow to your brain.