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If you easily getting tired while exercising and experience dizziness, there are a few possibilities you should look at. First is a heart issue that can be taken care of by an EKG, treadmill test and echo (this has been looked into). The next is anemia. The third cause could be low blood sugar or even a case of high sugar as in diabetes. Another cause is poor sweating as in Sjogren’s syndrome.
Of course if the exercise level is not systematically increased, one would get tired soon and it may cause dizziness and fainting as the blood vessels do not dilate as fast as required to increase the blood flow. Hence it is always wise to start exercising slowly and not push your limits to start with.
On the other hand it could be due to over exercising, or abruptly stopping exercise, or over hydration. Drinking too much water before, during or after exercise could also be the cause. Eat some salty snacks and see if it helps. Exercising on empty stomach which causes blood glucose to dip with exercise can also be the cause. One should exercise after 2 hours of a light meal.
I think you should discuss these possibilities with your doctor. It is difficult to comment beyond this without examining. A comprehensive investigation is required keeping all the points in mind. Hope this helps. Take care!