I just went through a big buga-boo with fatigue. Turns out I was anemic and had very low vitamin D. I was training for a marathon at the time, and it got so incredibly bad I had to abandon the marathon idea. I suffered for months, convinced I was just lazy and was just eating too much cr@p.
We're the same age, and it turns out active women our age are more likely to be low on iron... go figure? I'm sure your doctor will check your iron and your thyroid and your vitamin D. My iron and D were low enough that I ended up taking some pretty hefty supplements, diet alone wasn't going to cut it.
Took about 3 weeks, and then I felt wonderful and back to my old self.
Good luck!
"If Americans want to live the American Dream, they should go to Denmark." - Richard Wilkinson