Have you ever walked around the back of a truck and hit the trailer hitch with your shin? Take this, x10, inside your lower abdominal for however long it takes to get to the hospital. That's the best way I have of describing the pain.
When I was 4 my dad got his first one. It's one of my first memories. I remember taking him to the hospital. He honestly thought one of his organs failed/ruptured and might be dying.