The most common cause of a red rash that is itchy around the vagina is a candidal infection. This is also known as a yeast infection. A yeast infection often causes vaginal discharge that can be white and look something like cottage cheese. This type of infection can spread into the skin of the vagina and cause a candidal skin infection. The symptoms are usually red skin that can be very itchy. This type of infection is usually treated with oral anti-fungal medications such as fluconazole. Alternatively if the affected area is smaller, then you could use a topical medication. This would be up to your doctor.
Another possibility is that you have a bacterial skin infection also known as cellulitis. This is a more severe condition and requires antibiotics. Often people also report fever
and severe pain in the area; not so much itching. While this is less likely in you, it something that you would want to get ruled out.
I suggest that you schedule an appointment with your primary care physician
. He or she can exam that area and determine what it is you have. You should not require additional testing an you can get the appropriate treatment right there in the office.