The skin surrounding the external female genitals, the vulva, is very sensitive because it is designed to respond to sexual stimulation. This sensitivity can easily lead to vulva itch as a result of various stimulations outside of sex.
It is also situated in a part of the body that generates moisture and is without much airflow. If the air flow is cut off even more than usual by wearing tight clothes so that the skin gets even less ventilation, it is likely to protest by developing an extreme itch.
On the other hand, if the vulva is dried out by over frequent washing, especially with perfumed soaps, the skin will itch in rebellion against the interference to its normal environment.
Occasionally, the vulva develops cancer, in which case it will not only be red and itchy but hard to the touch.
Vulval itching is common in young girls before they begin their periods and in older women after menopause. It is thought to be related to the production of sex hormones, especially to the lower level of oestrogen present after menopause.
Women with diabetes also sometimes experience irritation in the genital area, because they are more likely to develop thrush and other infections.
Generally doctors prescribe a soothing cream and advise washing with a mild unscented soap once a day, or with no soap at all but using sorbolene or some similar substitute. Cotton underwear allows the skin to breathe. The area will probably be dry and sore as well as itching, and a lubricant during intercourse may be helpful.