This has been me for the last few years. There's Vitamin D, but also B12 and a few others. There's thyroid issues. Hormonal imbalances. Sleep issues. Immune system issues. Allergies, both environmental and food. The list goes on.
I just learned I have sleep apnea, about to get a CPAP. I'm really hoping this "fixes" me.
I'd go to the doctor. This can take a while if you get something rare, which is becoming more common. I was diagnosed with low testosterone at 27. It took months to diagnose, because it was so rare in someone my age. My doctor literally told me that was one of the things she thought of at our first meeting, but didn't test because of my age. Now I've met a few people my age with it.
Since this thread has been revived, I'll give an update.
Things got so bad for me that in July of 2018, I could no longer work. In January of 2019, I went to Mayo Clinic, because none of the local doctors had any idea. They've diagnosed me with Chronic Fatigue Syndrome, and the way to treat it is the generally healthy things we've mostly talked about on here. The one piece I was missing was mental relaxation, which is done through meditation or breathing exercises. I've been doing that everyday since then.
In May 2019, I was able to get back to work part time. I'm no longer on testosterone, my levels have gone back up to the low end of normal. And while my libido isn't where I want it, I'm mostly symptom free for the normal testosterone issues. My energy is still low, but is dramatically higher than it was in January of 2019, and it should keep going up over time.
With breathing exercises, the energy increase was very slow, it took months to notice much. What I did notice was my blood pressure would drop dramatically during it. When everything is quiet, I can hear my heart beating, and after about 8 minutes of the exercise, I could no longer hear it beating.
So if you're still experiencing these problems, you might want to try meditation or relaxation breathing techniques.
Best of luck to you!