Vitamin D deficiency can cause several disorders where fatigue is prominent, but it's not the most obvious cause of feeling tired. You may be slightly anaemic and the cure may be a handful of iron tablets. That's one of the most common causes. See the doc and ask for a blood test. Blood tests for anaemia are very common.
I suffer from tiredness and lethargy but since changing my diet (eating less and more healthily), plus regular exercise, I do not seem to be so tired. A long brisk walk seems to help, hard though it is to go out when feeling very tired!
As Yorkman suggests, check with your doctor. Also, I'm sure that "a handful of iron tablets" is purely a figure of speech but just in case others read this thread and take it literally, it is important not to take more that a recommended dose of iron as it is toxic in large quantities. See your doctor.
Definitely worth asking for a blood test. A routine blood test for me found I was deficient in B12 so now I have an injection every 3 months. If I don't I get very tired.
I've had chronic fatigue since I was 14 And I was only just told I had almost non existent Vit D levels. Overlooked by the other docs, no one told me it was consistently very very low. I'd been complaining of tiredness forever and no one thought to put two and two together.. I'd definitely invest I some supplements or getting out in the sun if your skin can handle it. I have dry skin and prone to heat rash/stroke so really don't like the sun... Makes my urticaria flare.