Hello, I Have Been Getting Pimples On My Vagina, They

Hello, I have been getting pimples on my vagina, they come one by one all different sizes... Iv gotten 3 since last year August the first one was about the size of a dime, the second one was a little smaller ,and the 3 one was the size of this "a" but it was painful when I touch it. The other two never heart at all. Please help me with what's going on with my body I am a sexually active teen. I got tested for HIV and I am clean but I'm not so sure about Std's