When you are very excited your skin can become covered with red spots. This is called the sex flush. Just before orgasm it can resemble a measles rash.
It appears as a result of blood flushing to the skin’s surface during excitement. In arises more easily in a warm environment.
With girls it normally starts above the belly button and spreads to the breasts, neck and face. With guys it is most visible on the chest, but can spread to the neck and the face. When the person is ready to climax it sometimes appears on the back, buttocks, thighs and arms.
About 50-75% of the girls gets a sex flush against 25% of the guys.
The sex flush normally disappears quickly from the extremities, but it can take a while before the places where it first appeared regain their normal colour.