Doc can't tell me why I'm always tired

Hey there,
Like many of you, I've been struggling with fatigue for awhile. Long story short, I was severely deficient in vitamin D and, around the beginning of the year, began a monthly regimen to get my counts back up.
It has helped and I'm no longer technically "deficient," but I am still almost always tired in spite of getting lots of rest. I went to a work conference nearly a month ago and haven't felt back to 100 percent since.
The other day, I was so tired, I just cried in my bed before breaking down and going to see my doctor for a second time.
I've been tested for all sorts of things - lyme disease, thyroid problems, etc, but he can't ever really find a reason for my fatigue.
How would I go about finding out if I have chronic fatigue? What are the criteria for a diagnosis?
A friend of mine who has fibro says a lot of my symptoms sound like hers began before she was diagnosed. But I don't have tender points or an overwhelming amount of pain.
I'm just tired of being tired.

I'm sorry to hear about what you are going through. I do have fibromayalgia and like you have been struggling lately with being tired every day. And like you it just came about one day, no real explanation why.I haven't been to my Dr yet about this new tiredness which at times can be pretty severe. I don't have health insurance and I get tired of people telling me that I just need more rest. I know when theres something going on that doesn't feel right as I'm sure you do as well. I wish you alot of luck and just wanted to write and tell you that your not alone

no test diagnoses cfs. Google cfs diagnosis. You have to be fatigued to a point for a minimum amount of time. Maybe you should se a different dr...one that is profecient in cfs/fibro.