It could also be an STI. Are you sexually active? do you have any strange colored discharge (discharge that isnt milky white or egg white, cm that yellow or green). Either way i would get checked out at your doctor. It could be anything from a yeast infection to a sti to a uti. Or you could just be dry and itchy.
It could be a number of things from being allergic to nylon panties (try wearing cotton only), or it could be from laundry detergent or garments not being rinsed properly, you could be allergic to the perfume in a product. I would check out allergy to a product or soap unless you have itching on the inside as well that may target a yeast infection and in that case you would need to check with a doctor to get at the cause of the itch.