Does sound suspicious of a Bartholin's gland cyst; will agree with Nannynormi.
I would NOT recommend self-treating it AT ALL; could aggravate the swelling.
Consult a physician, preferably a GYN physician ASAP.
Sounds like your bartholin gland may have a cyst. There's one on each side. I had it- needs antibiotics and lansing (not bad at all) If you don't treat it, like me, you end up having emergency surgery so you don't get the infection in your blood. Make an appt with a gynecologist and make sure you tell them you have a problem, otherwise they'll just give you an appt for the next month!
It could be a yeast infection. Only your doctor can tell you for sure, and if that's all it is (it was with me), he or she can give you a prescription that will clear it up quickly (if you follow directions completely). Until you can get to the doctor, you can try Tucks pads (basically cotton pads soaked in witch hazel). Sometimes they offer relief and can decrese swelling.... but they WON'T make it go away if it is a yeast infection.
Maybe underwear is tight or you have used a shower gel doesnt suit you, I would get it checked out with your Doctor