You might have an enlarged Bartholin Cyst. You have two Bartholin cysts, just to each side of the vaginal opening. These cysts can become enlarged and even become infected.
If the discomfort and swelling continues, you will need to see your gynecologist. Sometimes the cyst needs to be drained. If it is infected, antibiotics may be needed.
In the mean time, you might try some sitz baths, two or three times a day. Occasionally, the warm water from the sitz bath will cause the cyst to rupture and drain on its own. Acetaminophen or an NSAID (motrin, aleve, etc) can be used for the discomfort.
Again, the Bartholin cyst is the most common cause of a painful swelling in the labia. But, there are some other causes. This is why you will need to see your gynecologist.