Some days you might start out feeling great during your workout and then find that you are feeling like you are really dragging. There may be some valid reasons why this might be happening to you, according to Janet Hamilton, CSCS, an exercise physiologist with Running Strong in Atlanta. She offers nine possible causes for feeling sluggish during your workout.

1. Too much stress, not enough sleep

When your days are filled with stress and your nights are not filled with restful sleep, you will find that your workouts become more challenging. Hamilton explains, “When someone is struggling with their energy in the middle of a workout, the first thing to do is look at what her life has been like over the last few weeks.”

Make sure you are relaxing and getting enough sleep. Remember that working out does add an element of stress to your body, and this can combine with emotional stress and eventually take its toll on your body. Be sure you find the right balance for you.

2. Asthma and allergies

When you are struggling to breathe, you will feel quite sluggish during your workout. You will not be getting the oxygen that you desperately need. If you are finding that you are wheezing, coughing, feeling a tightness in your chest, or having shortness of breath during or after a workout, be sure to talk with your physician right away. You might be suffering from exercise-induced asthma, which means that your airway may react and become constricted in response to strenuous exercise.

3. Low pH

Pushing your body to the limits constantly, or operating all the time at your highest levels, can change the biological processes of your body. During high-intensity exercise, your body converts carbs into fuel, and makes byproducts including hydrogen ions. The more you push, the more these byproducts will build up in your system. This can cause your body’s pH to drop and your body to become more acidic. This will slow you down, and zap your energy, and ultimately make you less efficient. Make sure you are resting and recovering properly from strenuous workouts.

4. Workouts that are too intense

When you feel totally dead after every workout, it will eventually catch up with you. Hamilton explains, “If you ramp up your workouts too fast or don’t give yourself enough recovery time, you’ll feel like crap early on in a workout.” When this happens, make sure you allow your body to recover, with rest and some light workouts. Avoid doing more than two intense workouts in a row to help prevent this from happening.

5. Anemia

When you have anemia, your blood lacks enough oxygen-carrying red blood cells and its ability to receive oxygen is limited. Without proper oxygen, your cells cannot function properly. It is not uncommon for women who avoid meat and animal products to become anemic. If you feel fatigued, or suffer from insomnia, dizziness, leg cramps, pale skin or easy bruising, you might be anemic and should be tested by your doctor.

6. Dehydration

When you do not have enough fluids in your body, your blood becomes thicker and your heart will have to work harder to transport it and pump it where it needs to go. So, if you are sweating a lot, you need to replace the fluids.

Keep in mind you are losing much more than just fluids, you are losing vital electrolytes that help your muscle cells work properly. It can be helpful to weigh yourself before your workouts, and then again after your session. If you have lost more than 2% of your body weight, you are not taking in enough fluids.

7. Thyroid problems

People with low thyroid levels struggle with fatigue on a regular basis, and many people with thyroid issues are undiagnosed. When you do not have enough thyroid hormone in your system, your energy will tank, especially during your workouts.

You are also at risk for weight gain, depression, and muscle pain. It is estimated that one in eight women will have thyroid problems at some point in their lives, and many of them will not realize. So if you are feeling regularly fatigued, have your thyroid checked.

8. Too few carbs

Many women who are struggling with their weight tend to limit their carbs. You need to burn more calories than you take in if you want to lose weight, but you also need to consume enough calories to have the fuel to stay strong during a workout. If you are feeling sluggish, make sure you are getting not only enough calories, but enough carbs to keep your energy levels high.

9. Low glycogen levels

Stored glycogen is how your body fuels your muscles during workout. If you are following a low-carb diet, you might find that you need more glycogen in your system, not necessarily more sugar. If you run out of all of your glycogen while working out, you will become very fatigued.

Hamilton explains, “It’s like running to the supermarket rather than to your kitchen’s pantry for food.” Quick ways to increase your glycogen levels include snacking on simple carbs, sports drinks, or energy bars for a boost.