Small injuries. If you find that you are frequently getting injured during training, (i.E frequent small injuries, frequently straining a muscle, frequently pulling a hamstring), you may be over training, and you should rest your muscles more often. However, if you are stretching before and after exercise, and are injury-free with your training, you should be doing fine.
Fatigue/injuries. Overtraining results in recurrent injuries, fatigue, continued muscle aches, no performance improvement, &colds. When training for endurance, eat enough calories, drink plenty of water, sleep, and increase anti oxidants that become depleted.If you are still experiencing problems, take a break, see your doc, & visualize the game while your body recovers. Athletes in the zone perform better.