It could be your laundry detergent, especially fabric softener. When I stopped using softener on my underwear the itching down there also stopped. It could also be bacterial vaginosis (different from yeast. Or Trich) The only way to know for sure is for a dr to check you out.
I have the same problem. Except my labia swells. I already use sensitive everything, because this use to be a problem a year ago. Recently, I have been sick in bed. My labia was so itchy 24/7. I could not wear pant or panties. Now I am capable of wearing boxers, and baggy boy clothes. You might have to change your wardrobe. You should see your doctor to make sure you don't have a infection.
You might have estrogen deficiency.