I am sorry to tell you that most women here can give their opinions on what it is because you are asking for our advice but the best thing for you to do in your case is to definately see an GYN. We can't tell you what is wrong with you. Only the Dr can make that diagnosis. You do need to be tested for STD. I see that you are worried but we can't calm your fears. Please for your concern, see a Dr. It may be nothing but it will help you feel better in the long run. Hope everything turns out fine.
I wanted to add that even if you may not be sexually active, IT ONLY TAKES ONE TIME TO GET AN STD.
it could very well be an ingrown hair or what they call a bartholins cyst. i had that and man does it hurt. its like a pimple but hurts a lot. it began as red and di get a bit larger and was very purple. i went to the obgyn and she said it was that type of cyst and to put a hot cloth on it as many times as i could throughout the day. hopefully that will draw it out and feel relief. mine came back a few times over years(esp. when i had my period). look it up online to see if thats what you have ok? i hope i spelled it correctly but it should come up anyway..its common and non- threatning. just a nuisance.