Researchers say sex can trigger what is called transient global amnesia as can other physically strenuous activities.
People in their 50s and 60s are the most likely to experience an episode.
The closest researchers have come to explaining this sex-triggered amnesia is that the problem may not be with the brain but can be found in the neck instead.
Apparently, some people have an insufficiency of the valves in the jugular vein.
Strenuous activities increase pressure and impact blood flow.
The insufficient valves may allow deoxygenated blood to push back up the neck and into brain regions key to memory formation.