Hi, the itching could be caused by many things...yeast infections and BV infections are very common, and can just pop up for no reason. Also, using scented tampons or pads can cause reactions in some women, as can certain dyes or perfumes in detergents and soaps. Has she switched to new brand of any of these?
Also, I just wanted to say that even virgins are at risk for some STD's, especially considering what some people consider a "virgin". STD's can be caught by oral sex, and some viral STD's, such as herpes, molluscum, and HPV (warts), is transmitted by genital skin to skin touching...no actual sex needed. Also, over 80% of the population is already infected with oral herpes (HSV1) by the time they reach 14 years of age...most being infected while they are still children, through non-sexual means. Only about 20% of those infected ever actually get the coldsores, but just carry it and can spread it. Oral herpes (HSV1) can then be transmitted to a partner's genitals through oral sex....thus meaning that a couple who are virgins with absolutely no other sexual contact, can still be at risk for genital herpes.
Now, don't get me wrong, I absolutely do NOT think your girlfriend has herpes, I am sure it is just a yeast infection or something like that...many of us women are plagued by these infections all the time. I just wanted to clarify that it is possible for an STD to pop up in a relationship with two virgins.