Answer:
You'd better see a doctor for examination and diagnosis. It could just be a rash, but it's better to get the right treatment than to make guesses.
It sounds like vaginitis. Best to get a pelvic exam to rule out any infection. Vaginitis can result from an STD, but not in all cases. First, don't panic. Get an exam and find out what is going on. If you are uninsured, contact your local Planned Parenthood as they have sliding scales and scholarships available.
In the meantime, use some tea tree oil to help with the inflammation and itching. Also try some Boudreaux's Butt Cream--for babies--as it helps dry up rashes and takes the itch out. Best to leave the rash alone--the more you scratch, the worse it gets. Also abstain from sex until you are healed.
When you get to your appointment, ask about latex sensitivity or sensitivity to spermacides if you are using condoms. There are alternatives. Sometimes a rash can result after being exposed to these products.
Being sexually active means getting more pelvic exams, at least once a year and getting tested and being protected. If you do come down with something, don't freak out--best to know what to do to get treatment and to protect your partner and your future partners.
HTH and good luck.