What Causes Rash And Itching In The Vagina Before Menstruation?

i am a female aged 24.just the night before i had my monthly period,my vagina started itching.ths went on throughout my mestruation period and even after.i have noticed a rash on the inside part of my clitoris.this is the first time i have experienced this.i am sexually active bt i last indulged 3 months ago.what could be the cause