The most common cause I can think of in your case is infection of the bartholin's glands. It may also be STDs like Chlamydia, chancroid
, granuloma inguinale
, gonorrheoa or even syphilis
I would like to know if the swelling is painful or painless in coming to a conclusion.
You need to undergo STD screening. I understand that you have taken high dose antibiotics earlier. However, treatment without determining the cause of the symptom is of no use.
Get a culture and sensitivity of the pus done to detect the organism and get treated accordingly.
I wish you a speedy recovery.
Hope this helps.