You are pushing your cardiovascular sysyem (heart) farther than it is used to going. If we push ourselves beyond the level in which we regularly go then it exhausts us tremendously. I have clients that exercise for 20 min and are just beat. So it is relative in comparison to your fitness level.
Secondary, look at your diet. Energy is mostly coming from carbohydrates so ex. client eating only salad with chicken (protein) comes to the gym and wants to faint within 10 min because she ate almost no carbs. Carbs 50-60% of your diet, Protein 12-20%, fat 25-30% of your diet daily.
Thirdly, other health conditions can apply to your energy levels like Vitamin D levels. If your D levels are low then your body is extremely lethargic. There are many other illnesses related or relevant.
Mostly, look at your diet, check Vitamin D with Doctor, and increase exercise level slowly exercise does exhaust everyone a certain amount.
I hope someone can answer your question. When I get similar symptoms the doctor has looked at blood sugar (which was not the problem) and besides, the symptoms SHOULD go away- not leave me feeling bad ALL day. Nutritional issue maybe? Try eating several bananas or peanut butter or other foods that are high in one nutrient or another and see if that shortens the symptom time. Good vitamins for active women from GNC helped me for a while, but no more.