Itching inside vulva. Why this itching?

Why i have consistence itching inside vulva , mainly inside labia minora , i do clean my vulva regularly with plain tap water or with soap sonetimes , i always wash my vulva with tap water whenever i pee/poo , i do change my underwear everday , so why this itching? Sometimes i think it is happening because of pubic hair or due to tap water , but everybody has pubic hair and i think most people wash their genitals with normal tap water  , and i dont have any pimple or anything in my vulva, onlyy this itching irritate me everyday , plus my periods are also irregular from the starting since first menarche , please tell me possible reason of this itching problem and please guide me what should i do to tackle this problem ,