What about Bartholin's Cyst??? Look it up! Sounds like a possibility to me.
I cannot make a specific diagnosis on line, but it sounds like an infection of some sort. Given the onset a few weeks after a possibly high-risk sexual encounter, an STD sounds possible; gonorrhea, chlamydia, herpes, and syphilis all come to mind. See a health care provider soon. If you are not confident in your own health care provider's knowledge about STDs, consider visiting your local health department STD clinic, or a reproductive health clinic such as Planned Parenthood. In addiion, if you know how to contact your partner, you should do so; find out whether he (or she) has any STD, any symptoms, etc.
Good luck-- HHH, MD