You may have a yeast infection. See your GYN for cultures. 

Reason/cause very well could depend on year age. For younger premenopausal women even though you are not having any discharge most likely culprit would be a yeast infection. Could try over-the-counter vaginal product to see if that might help or check with your gynecologic provider to see if they might allow a course of Diflucan oral medication or pelvic exam in office to check for yeast or other possible causes. Older women (postmenopausal) sometime suffer from a problem called vulvar dystrophy/lichen sclerosis which can cause chronic redness and itching and sometimes swelling. Always possible you are allergic to something you have been coming in contact with in the vaginal/vulvar area. Over-the-counter hydrocortisone might help that. Sometimes stronger prescription creams are utilized. If something focal or localized would have checked.