Overtraining
Overtraining refers to the accumulation of too much stress (training or other types of stress) that leads to extreme fatigue (physical and mental). Overtraining occurs when overreaching is carried too far without adequate recovery. With overtraining, the athlete has dug him/herself a hole that may take several weeks to get out of. Extreme cases can be season ending. Overtraining is obviously to be avoided and the administration of an appropriate training plan can help the athlete use the overload principle for performance gains without overtraining.
If you are lethargic, have heavy legs or difficulty elevating your heart rate during training at increased intensity levels; then these are signs you are overtraining. If these signs continue for more than a day; then take a few days off completely followed by a few days of short, easy workouts before returning to your regular program.