What Causes A Pimple On The Top Of Vagina?

I have a small pimple looking thing on the top of my vagina it s abit itchy and has been there for a long time ? I m not very worried that it is a Sti as I had a Pap smear not long ago and she said anything was ok and she saw the pimple and said just keep an eye on it and call of u get anyone but there is still only one. Can I get some sort of cream for it