File this under, "Too much of a good thing." Exercise generally has tremendous health benefits, but moderation is equally important. Overdo it, in terms of either intensity or duration, and you run the risk of putting your body under undue physical stress.
That can have consequences that go beyond repetitive strain injuries, such as shin splints or knee pain. Like mental or emotional anxiety, an unhealthy amount of physical stress forces your body to protect itself, and preventing ovulation can be a by-product of that phenomenon.
This form of secondary amenorrhea is often seen in endurance athletes, such as cross-country runners or professional cyclists (typically, their lower body-fat content also plays a role in delayed menstruation).
Coincidentally, the same condition can be experienced by women who suffer from a chronic illness that, on the surface, may not appear to be related to their menstrual cycle.
A late period could indicate other serious medical conditions. Keep reading.