It could either be razor burn or ingrown hairs.
If you might think it's ingrown hairs (you can look in a mirror really closely and see if there's a hair in there), get a damp cloth with warm water and push it up where the bumps are. It'll soften the hairs and make them easier to come out on their own.
If it's razor burn, you can read a great and short wikiHow at this link
Razor burn is a rash or skin irritation. You can also use aloe vera gel down there which moisturizes and is great for skin irritations.