Hair follicle irritation. Extremely common. Not herpes related if it happens every time you shave. Don't scratch, that can lead to infection.
You might want to try a different way of hair removal too. waxing might be better for you or even consider laser hair removal. You can also just trim hair shorter with a body hair trimmer instead of taking it all off. There are also many post bikini area shaving products out there too. The skin in the genital area is very sensitive and needs treated with extra tlc!!!!