What Causes Itching In Vaginal Area?

I have been having severe itching on my vulva and outside of my vagina for about 3 weeks now. I did a 10 day over the counter yeast infection medication (suppository), and used the cream and ate lots of yogurt, but it's still here. I'm also 20 years old and sexually active in a monogamous relationship, and have a lowered immune system because of a medication I'm taking. I know this isn't a STD of any sort, but it seems a bit long to still have a yeast infection. Could this be something else? Should I see a doctor?