A certain amount of breathlessness during exercise is normal, especially when engaging in a sport as demanding as running. Increased activity leads to faster breathing, which brings oxygen into the body to help lower building levels of carbon dioxide in the body during exercise. Once a normal level of carbon dioxide is reached, breathing usually returns to normal. Issues related to shortness of breath while running can be addressed by tweaking a routine slightly. When shortness of breath occurs suddenly or continues even after slowing down, it could be a sign of something more serious, so consult a doctor if symptoms don't improve after making changes to your exercise regimen.