This happened to me once in my lifetime. Do you wear synthetic underwear?
Honestly, it was because I was wearing synthetic underwear. My mom bought me really cute polyester underwear for Christmas, and I wore them a lot because they were new and nice.
What happens with synthetic underwear is that your pee and poop just stay there. Synthetic underwear does not absorb pee; you just sit in it all day. Your tissues get irritated, and when you scratch your irritated tissue pee gets in it. Essentially it is like an infection you have.
Switch to cotton underwear and use a very mild soap to wash yourself. Keep it dry. Tucks pads are have witch hazel in them, a natural cleanser. Take a Tucks pad, pee, wipe yourself with a Tucks pad, follow it up with toilet paper. Then take a clean Tucks pad and place it on the affected labia. Do this every time you go to the bathroom.
I hope that helps you!