Giddiness, tiredness, confusion.

When I walk or do any physical work continuously for more than a minute i get very giddy, tired, fast heart rate, heavy breathing, a sense of confusion. I came across this problem few years back when I was in a dance school and I was gyming together and now i have encountered the same problem after trying to start walking as an exercise. I have taken all the tests for any deficiencies or problem like thyroid but the results are fine except for vitamin d deficiency, I have completed medication for that. I would want to know what my problem is as no one is able to explain it clearly. This problem doesn't happen only while walking but even if I clean the house or stand and cook for half an hour without break, paint that is any form of physical activity.