Working out can be incredibly rewarding, fun, and challenging, but everyone has off days when your body refuses to cooperate. You muddle through your workout or just give up and go home. Get to the core of what's happening, how to deal with it when it's happening, and how to prevent future episodes.

Symptoms: Dizzy, lightheaded, nauseous, totally winded, overheated, overly high heart rate.
There are three primary reasons why you may be feeling like this.
1. You're intensity level is too high for your fitness level. Slow it down a bit but continue moving. If you're doing high intensity training walk around for a bit to let your heart rate come down or jump on a lifecycle and pedal at the lowest level for 5 minutes. When you feel o.k. continue what you're doing at a lower intensity (less weight, less reps, slower, etc.). In the future try starting out slower then increasing as you go.
2. You've had nothing to eat in hours or you're totally dehydrated. Grab a snack bar and a big glass of water. Eat this while you walk around then get back to work at a lower intensity. Always eat a balanced meal 1-2 hours before working out to avoid this.
3. You had a ton of caffeine and or ephedra and not much to eat before your workout. In this situation you can also eat a snack bar, drink water, then continue at a lower intensity but this doesn't always help. Some individuals are very sensitive to caffeine/ephedra so it's best to call it a day and stop otherwise you could end up vomiting, passing out, having a panic attack, or worst heart palpitations that feel like a heart attack. Don't take any chances and just stop while you're ahead. Try working out without supplements for a while and make your body respond with alternative training techniques.
4. You didn't warmp up for ten minutes prior to your workout. Warming up is crucial to having a productive workout. It's gets blood flowing through your system, your joints are properly lubricated, and it mentally prepares you for hard work. Skpping a warmup to save time will only cause you to get sick or worst to get injured so just do it every time.

Symptoms: Sleepy, fatigued, totally winded, weak
One of three things is wrong.
1. You're sleep deprived and your priority at this point should be to get rest not workout. I've worked with several clients who had night shift jobs that wanted to workout after work, and fatigue was always a huge problem. If you're body is fatigued it's impossible to train properly so always get rest first or you may end with an injury.
2. You didn't eat enough carbs or you ate the wrong kind of carbs before your workout. Everyone these days is cutting carbs to lose weight but guess what?, when you workout hard you need carbs to fuel your workout and burn fat. Try eating some type of starchy carb right away especially if you're lifting heavy and in the future eat a combination of complex and simple carbs 1-2 hours before working out. ie.slice of whole grain bread, tbsp almond butter, 1/2 apple.
3. You're overtraining and you need stop before you get seriously injured. Stop training body parts twice a week and doing marathon cardio sessions twice a day 5 days a week. If you learn to workout correctly you don't need to spend hours a day working out. Get tips on how to workout smarter not longer and you'll get the results you want in less time.

Symptoms: Cramps, aches, pains, muscle fatigue
There are numerous reasons you may be feeling this way including: lack of sleep, hangover, menstruation, lack of warmp-up, you're on the verge of injury, stress, and dehydration. Stop your workout, drink some water, and do 10 minutes of light cardio plus another 10 minutes of light stretching. Call it a day and determine which one of the reasons above was causing the problem so you can address it next time you workout. If you're dealing with a potential injury seeking the help of a sports massage therapist can be beneficial to avoid serious damage.

Amazing resource for the serious workout junkie: Exercise Physiology: Energy, Nutrition, and Human Performance
The seven chapters discuss nutrition as the base for human performance, energy for physical activity, systems of energy delivery and utilization, enhancement of energy capacity, the effect of environmental stress, the effect of exercise on successful aging and disease prevention, and body composition, energy balance, and weight control.