Ask your doctor about your Vitamin D levels. After I had radiation, I was super tired, but once I started taking a Vitamin D supplement, within 2 days it was like all the lights turned back on and I felt like my pre-treatment self. My levels were well below 40 (The test is called a "Vitamin D 25 O-H" and your minimum level should be 40. Again, please check with your doctor.), but by taking Vitamin D supplements daily, my levels are now in the normal range.
You can read more about Vitamin D and its benefits: