PLEASE HELP. sore on vag and don't know what?

I've had a sore on my vagina for two days now, it's about the size of a fingernail. It's mainly white but has a blood red bit on it and a purple bit on the edge. It's quite swollen and I don't know what to do. The top has come off it a little and it stings really bad sometimes. I'm only quite young so please tell me what it might be and how long it will stay.