What Causes Darkening And Itching In The Area Around Thighs And Vagina?

Hi, i am 25 years of age and weigh around 75 kgs, my height being 5.4". i have this irriating problem. my skin between the thighs and around the vagina is getting darker which is spreading towards the anus area. the skin in the darken area is smooth but very itchy. it disturbs my sleep. i have tried applying anti-fungal powder but of no use. its been almost a month. the itching stops and comes back when the skin starts getting drier and turns white in colour because of the dryness.