It's unlikely to be herpes - herpes isn't pimples.
Have you seen a doctor?
When you get herpes from oral sex, you get genital hsv1. You could already have hsv1 - 67% of people under 50 have it globally, and the vast majority never get symptoms. If you have it already, you won't get it again genitally.
Since it had a dark spot around it - I'm guessing like a ring? - it sounds like it might have been infected. If it still looks like that, get it checked out.
Also this at the bottom of my back right before my buttcrack