sounds like another boil to me. I had one, it got pretty big - like yours - and it hurt like crazy. Once is ruptures, you're instantly pain free. Hot baths or hot compresses bring them to a head quicker and are soothing.
You said you aren't sexually active. You HAVE HAD SEX THOUGH, RIGHT? Just not lately, right? And have you received any oral satisfaction, and considered it "not sexually active?"
If you receive oral gratification, even if the person looks like there's nothing there on their mouth; they could be just about to get, or just getting over a "cold sore" aka "fever blister" which is really Herpes simplex 1 virus. If such a person touches you down there with the affected area of their mouth, you can then contract genital herpes.
Also, if YOU have the simplex, and you just so happened to play with yourself after you had recently touched your mouth, or even had to change a tampon, (in some way getting your saliva or touched the area of your mouth then used the same part of your hand on yourself.) you may have given it to yourself.
Away from the scaring: Did you do something lately, like change a tampon, and maybe catch the edge of your lip (labia) in the edge of the applicator, as you were inserting a new tampon? Anything like that you may have forgotten?
What about irritating your sensitive mucous glands w/ an outside substance? Example: maybe you ate or touched something spicy w/ hot pepper or horseradish, spicy doritos, spicy brown mustard, etc. I had something like that happen to me once. It sounded just like you explained. I really thought I had herpes and freaked on my kid's dad b/c I thought he gave it to me. I still don't know if it was the spicy sweet chili doritos I ate right before I had to run to the bathroom, or a really bad yeast infection: Don't believe a yeast infection can do something like what you described: Google "yeast infections" and click on "images!"
WAIT!!! . . . YOU CAN GET A BOIL AKA ABSCESCE ON YOUR VAGINA?!?!?!?! ARE YOU REALLY SERIOUS?!?!?!
Okay. Thank you for the help. I'll be sure to take a warm bath everyday instead of showers.