What Causes Bump On The Vagina During Menstruation?

Every month during menstation, there is a pea sized bump on my vagina's outer lip. Its pink in color. It swells during menstration, then shrinks after. It also itches a lot! Trichanomis is the only STD a doctor has ever diagnosed and treated, and I get tested every 6-8 months for everything. This has been going on for at least 5 years now. What could this possibly be?