Vitamin D plays very important roles in human body. Its deficiency can be due to inadequate sunlight exposure and inadequate intake. Liver and kidney problems can hamper the absorption of vitamin D in body. Symptoms of vitamin D deficiency are bones pain, frequent fractures, bone deformities etc, So, feeling tired is not directly related with Vitamin D deficiency. Tiredness is condition which can be due to thousands of reasons. In my opinion, you should visit your problem with physician not with GP.
Yes, it can make you tired. I just saw my doc because I get migraines. I told them I was really tired all the time. They checked my vitamin d and it was at 19. They said that could be the reason I am so tired, since all my other tests came back normal. I am starting to take the vitamin d supplement now. We'll see if it works!