Sounds to me like a Bartholins Cyst. I just had one of these a few weeks ago. From everything I have read on them, they are pretty common...caused by bacteria blocking the gland on the side of the vaginal opening. If you look it up, it will tell you that if it isn't horribly sore, to do sitz-baths (sit in shallow, very warm water for 8-10 minutes for 3-4 days). That should help it drain. I know, gross, right? Of course, this is just what I have read and done myself, and it goes away pretty quickly. If it continues to swell, or if the lump doesn't go away after a few days of the home treatments, and if it is extremely uncomfy, it might have to be drained. I'm not a doc though, so of course, I woud say check with them. But, if you feel interested, check out Bartholins Cyst on google...there is tons of info. Might help.