It's quite probably going to be some sort of cyst. At any rate (and I know how easy this is to say, and how hard to do!) try not to worry too much -- the chances that it is anything very serious are very, very low. The first thing that came to mind for me as well was a Bartholin's cyst, (http://www.vaginapagina.com/index.php?title=Vulvar_Bumps/Cysts#Bartholin.27s_Cysts) but a doctor will be able to tell you for sure.