May be a yeast infection.
SEE YOUR DOCTOR... sex will probably cause it to flare up. So does antibiotics (which, by the way can make oral birth control not work!)
FOR TEMPORARY RELIEF... try sitting in a bath of water and white vinegar.
Try tea tree oil, rub on with a cotton ball. Tea tree oil is a natural antiseptic.
WASH HANDS WELL! Don't touch the bottle or anything after touching the infected area, without washing hands (this should apply throughout life!)
keep itchy area clean and DRY, try sleeping in baggy shorts, keep cool and dry and aired out.
I AM NOT A DOCTOR. GO SEE ONE.
Some things, if untreated can lead to infertility, or worse.
AN OUNCE OF PREVENTION IS WORTH A POUND OF CURE!