When you overexercise, you run the risk of becoming dehydrated. This leads to an electrolyte imbalance, which can cause cramping, heart palpitations, nausea, vomiting and passing out.

To stay hydrated, drink 17-20 oz of water an hour before your workout. While exercising, consume 7-10 oz of water ever 10-20 minutes, as you are actively losing water through sweat. You should also drink eight ounces of water right before and right after your workout.c 

Overexercising means you're using your muscles beyond what they are capable of and completely depleting their glycogen stores, possibly to the point where your muscles will burn themselves as fuel because it has exhausted the glycogen and fat stores. This can result in injury from pulling or straining muscles.

Too much exercise can lead to mental and physical burnout to the point where you won't or can't go to the gym to work out. Exercise shouldn't feel like torture; you should want to do it.