having the same problem woundering what causes it..............
Where exactly is it? On the inner lips, or on the outer area, or on the pubic bone area?
It could just be an ingrown hair, or a sebacceous gland now turned into a small pimple. They are very common.
If on the inner lips, it may become bigger and harder to get rid of. They can hurt terribly and turn into a cyst if not treated correctly. The area there is dark and moist, which can cause it to not heal up quickly.
If on the other, you can just wait for it to go away or use a hot compress (hot face washer folded) and see if that helps to open it up to remove the pus.
It is possible however, that it could be an STD. If you are concerned it is always wise to see a doctor, as many of the STD's are treatable in early stages.