What Causes Lump On Vaginal Opening?

hi,, i am 28 yers . i have a lump in the openin of my vagina.. few days back been to gyne and was given anti bitotics for infection.but now i dont feel burning sensation but lump still ther .she said as no pain not to worry. this lump is sometimes big and sometimes small . i fel worried as it is in the openin of vagina and it feels bigger inside. Please suggest wht i need to do