Welcome to HealthcareMagic. One of the causes could be contact dermatitis
or allergic reaction
to the soap, cosmetics or even food or texture of clothes. It can be that the area has come in contact with an allergen which can cause such itching. You can apply some calamine
lotion at the rash
as it will help in soothing the skin. Also wear cotton clothes. Application of mild corticosteroid cream and antiallergic medicines may help with the itching. Hope it helps you. Regards.