How in the world can a light skined woman who works outside get a vit. d deficiancy?

ok I went to the doctors and had the complaints of sore joints and pain. The doc ran blood and found to be deficiant in vit d. I am usaully absent minded about puting sunscreen on and i get alot of exposure to the sun. Is it a farce that one can absorb vit d thru the sun? If so then how.