Could be an ingrown hair, or a minor abscess, a boil...It doesn't sound like an STD to me.
If you're brave you can try draining it...You'd either have to pop it like a pimple, or lance it with a pin. But ALWAYS be very sterile. Wash your hands really well, disinfect anything going near it.
Or you could go to a doctor and get their opinion if it doesn't go away within 3 days.
Ive had one of these, I got into a hot shower and gritted my teeth and squeezed it..it was extremely painful but busted and healed quickly. I found it came out there after i had squeezed a round nickel size one on my bikini line. My friend had these also, she were told they were everrything from boils to ingrown hairs.And didnt go away after years they looked like perminent bruised lumps .But after seeing a city specialist for othe problems he said of you have..... (cant remeber the name) they form when you have excessive hormones in that area and are often an indication of stress or other problems etc with your hormone levels...Hers were medical and as i have found out so are mine..I have a huge amount of abnormal cells, which I am about to have day surgery for. So if yourbump came from one of these lumps please go and have a pap smear and have your hormone levels checked out.