Hmmm That could be a number of things depending. If you've been chafing alot down there (maybe new panties or bottoms of some sort that don't breath well, or even sitting too long getting hot and sweaty down there) then I would assume it's a boil and to take care of that would be to wash it with antibacterial soap and keep it cool and dry til it pops on it's own. Which could take a few days don't force it though, and once it does pop be sure to wash your whole leg and outside pelvic area (not inside your vagina either just where it popped) with antibacterial soap or it will spread. If its not a boil (I would research it more if I were you just to be sure it IS in fact a boil BEFORE treating it) then idk it's probably infection of some sort and depending on if its viral or bacterial you treat it differently and the only thing I can tell you is to see a doctor to diagnose proper treatment.