You likely have what is called a "bartholin gland cyst.
" the bartholin glands produce mucus to help defend the vulva against infection, but sometimes can get obsturcted in a similar manner to a pimple. While this is a likely explaination, you should see a physican to make sure it isn't something more serious. The best initial treatment would be a "sitz bath
" (warm water 2-3x day).
Answer assisted by Michael Rommen, Medical Student