if you are low in vit D then you are probably low in calcium too as you need vit D to absorb calcium. Calcium is not tested as part of regular blood tests, it is a specific test that has to be requested.
low calcium will leave you with low stamina as it affects your bones, muscles and the circulation of your blood too.
Try taking a thirty minute walk every day, keeping as much of your skin uncovered as possible as vit D is got from sunlight and is absorbed through the skin. sun cream will block the absorption of vit D.