it could also be cancer, MS, depression, fibromyalgia, CFS, blood sugar problems (diabetes), thyroid problems, sleep apnea, lyme disease, any type of autoimmune disorder, like celiac, and allergy, actually pretty much any disease could cause your symptoms
source: i'm a hypochondriac
i'd start with a CBC (complete blood count), it's a very simple, standard test--it'll help point out major infections, gives a slight clue about whether you have blood cancers (not its intended purpose though), checks for anemia, and probably other things i don't know about
you might also want to test your blood glucose--you can watch for coupons and get the monitors for free sometimes (make sure to get one with free test strips inside, they are what cost a fortune)--you can do your own at home fasting and challenge blood glucose tests (test how fast your body lowers its blood sugar after consuming sugar)
a liver panel could be a good idea too, you would feel sluggish if that were impaired
BTW, you can get vitamin d from fish, and many products are fortified with vitamin d, especially milk and soy milk, yogurts, some orange juices, etc. oh yes, and the sun as others have mentioned