What Causes Swollen Vulva?

within 48 hours one of my vulva has swollen up 3 to 4 times normal size. Very red hard in center. Very painful Could this be a cyst or boil. What is the best place to seek medical attention, E R or wlk in doctor clinic. I have insurance but only certain Drs are in HMO and since this is a holiday weekend, I really can't wait.