What Causes Dryness And Itching In The Vagina?

Hi, I am 19 years old and sexually active. Recently, my vaginal opening became very itchy, to the point of discomfort. Then I began to not that the inside of my labia was very dried out. I have been using Vagisil for three days and noticed it helps a little with the itchiness. Could this be an STD or yeast infection and what should I do? Thank you!!