Did you change soaps ? It maybe because of that or it could be the fabric of your underwear or you underwear could be to tight. Go to the doctor asap if you can to have it checked out
sounds like maybe a yeast infection. did you rub or scratch the itch? if you did that enough the swelling would occur. if you have never had a yeast infection you should go to the dr.
Is it an old razor? Could be razor burn-that area is super sensitive. If you've scratched or rubbed it at all it could cause swelling. But if it doesn't clear up quick I'd see a dr. Good luck!