you're not alone... I have had that a couple of times with this bump - have swollen but then gone back to normal within a few hours... Think it could either be fluid retention, heat or just pressure - maybe depends how LO is sitting?!
On the august 2007 antenatal clubs there has been mention by a few women of developing "porno flaps" mid pregnancy! I think it's probably a very normal part of pregnancy since we have like 30% extra fluid/blood circulating at that point, and it's got to go somewhere!
I wouldn't worry unless it feels heavy/sore/achy/itchy as if it does it's possible it could be vulval varicose veins. lovely!
I wish there was a rule forbidding anyone to apologise in advance for the "TMI"-ness of their questions on mumsnet - surely the whole point of this place, being a largely anonymous internet forum peopled almost entirely by women with direct experience of pregnancy and the occasional yuckyness thereof - is that you should feel able to ask questions about slightly squirmy stuff you couldn't chat about at work or over dinner at home. I can think of very few things that really would be TMI in this context, and swollen ladybits ain't one of them.
(as others have said, that's entirely normal btw- something to do with increased blood flow in pregnancy.)