Cheryl, my wife the runner, has been dealing with an injury for the past several months. It happened at a race and the initial signs were faint. However, as she attempted to return to her normal running schedule, she immediately got a slightly firmer message that things were not as they should be.
This brings up the question: When should you listen to your body? As children we are told by parents and teachers to "Stop crying! You're not even hurt!" When you obviously are! Your skinned knee or bruised elbow tells another tale. And there is no denying the pain message your body is sending to your brain. A message you can understand loud and clear as a child.
Lastly: Experience! Newbie runners may push themselves too far, too fast. They might lack the experience to know the difference between normal training pain and "something is messed up" pain. To them, all runs probably feel like "I'm about to die!" pain. As you run longer miles, inevitably you will learn to distinguish between the two. And, if you are lucky, you may forever avoid any serious or reoccurring injury from ever happening.