Although rare, fainting or feeling faint during or after an orgasm is possible. Known by some as Le Petit Mort, "the little death," this phrase was used by the French to describe the temporary loss of consciousness during orgasm. While this is not the norm, there are some logical explanations as to why this can occur. As secual arousaloccurs, your breath shortens and quickens. For some, their breath becomes so rapid that they hyperventilate. When this happens, the normal levels of carbon dioxide in the blood fall, which may cause dizziness, lightheadedness, or actually result in fainting.
Another explanation is that the brain doesn't receive enough oxygen because of reduced blood flow. This happens because sexual arousal causes blood to flow from the brain to the genitals. This may produce an altered state of awareness or a true loss of consciousness.
Fainting after orgasm could also be a symptom of a serious condition, such as an abnormal heart rhythm. During daily activities, people with this condition may feel fine. However, when they exert themselves during exercise or sexual activity, their heart can't maintain adequate blood pressure. When the exertion ends, their heart rate decreases, but the blood vessels from the muscles remain dilated, causing them to feel faint or pass out.
I would suggest that you havea EKG to rule out abnormal haeart rhythmn.