What's your resting heart rate? If its greater than 10 beats per minute over normal then take a day off, your probably over exercising

Feeling weak? If your unable to get in the car and do up the seat belt without feeling it, your body is telling your to take a day off the exercise.

On your 4th cold this month? Again take a rest for a couple of days.

How did you sleep last night? Not well? Could also be down to over exercising. Over aggressive? Over exercising.