It could be anything though :) thats just what I have experienced. My primary care doctor was able to do labs and such and get everything worked out. I would insist on going in and getting all the labs done
You might want to get bloodwork done, Myth. I had something similar happen a few years ago. Was sleeping 18 hours a day every day and still exhausted. Had a severe iron/B12/ and magnesium deficiency. The magnesium was so low my doctor had never seen numbers like that before. You might be missing something, and that's why you're feeling that way :(