I'm not sure if you're aware but it is possible to get pimples down there, I've had a few myself and most of them have been painless and have taken a long time to come to a head, if that doesn't seem like that could be the case, have a look into bartholin gland cysts, a quick google search will give you some info. There's lots of oil glands in the area that can get clogged or irritated and swell. Never be afraid to go to a doctor with things like this, they deal with them daily I'm sure, and it's better to be safe :)