What Causes Vaginal Nevi With Pus And Pain?

for a number of years i have had spots filled with pus on appear on my vagine. They come just before my period is due, but not evey month. The spots hurt when i scratch them, but dont cause too much discomfort. When I was about 22 (39 now) i had unprotected sex with a man who I now know had lots of unprotected sex with lots of people. I am worried that this is an STD, but have never checked it out