It is highly possible that during sex your blood pressure increases significantly. While this is happening, veins in your head become enlarged. One vein that runs behind the ear could be enlarged to the point that it applies pressure to the fluids in the inner ear.
You could also be experiencing a side effect from “viagra” or similar sexual enhancement drugs that are known to affect your inner ear.
Positional vertigo can also happen if you are lying horizontal during vigorous sexual activity and then stand up/sit up after. The sudden change in orientation of your head can cause you to feel dizzy and possibly even faintbut just for a second.
If you start feeling this more often than just after sex, you could have an inner ear infection or something else causing problems. See a doctor if it becomes too frequent to manage.
This answer originally appeared on this Quora question on Vertigo Dizzy.