What Causes Stinging Sensation In Vaginal Opening?

All of a sudden the bottom part of the opening of my vagina stings when I whip it after I pee and it stings if I touch it in the shower. It does not itch and I have my normal amount of odorless discharge. I just woke up one day and it stung to whip, also there is no swelling and it is a tiny bit red. It has done this like twice before as well. What can I put on it and what could it be?