As I was doing my CME (continuing medical education) last year I read that fully ONE HALF of all unaccounted-for (not associated with trauma, overuse, etc) joint pain and muscle pain is due to Vitamin D deficiency. That is a LOT of cases! I have learned that it is impossible as a doctor to predict what someone's Vitamin D level is without doing a blood test. We used to think that patients who got 1/4 hour of sun exposure every day got enough Vitamin D, but I have not found that to be true. Also, some patients are not choosing foods which are fortified with Vitamin D. Even if they are, if their levels have been low for many years they cannot catch up simply by more sun and more milk intake.
Low Vitamin D levels definitely contribute to thin bones (osteoporosis, often resulting in bone fractures ) and sometimes can contribute to low energy levels, and decreased mental concentration. Just in case you get too excited, though, and start consuming LARGE doses of Vitamin D, if levels are too high this can contribute to the development of kidney stones!
Seeing a doctor can help to prevent either extreme.