Suggest Treatment For A Painful Lump On The Vagina

I just noticed a single crater-like sore just around the opening of my vagina, not quite inside. It is a little bit bigger than a pea, and is annoyingly painful. I researched herpes, but all the pictures had multiple sores, that looked more like blisters to me than craters. This is the third day that I've had it. I'm wondering if someone has an idea what I have?