Cortisol is a stress hormone, consider the following:
Too much cortisol can be a symptom of overtraing and can potentially contribute to immunity function.
Cortisol also helps the body burn fat and maintain blood glucose during extended periods of exercise.
High levels of cortisol have been associated with illness and fatigue.
If you have been feeling run down and training intensively, maybe cortisol control is in order, scale back the intensity.