What Causes A Round Bump On The Genitals?

I took an antibody test four weeks after the date i could have been exposed to HIV, i came out negative. Two weeks after the date i could have been exposed i had a sore throat. Around 3-4 weeks i got a rash on chest that look like dark purplish spots. I am dark-colored though. I found a round bump on my genitals. Which std could this be?