Your body has been through a LOT during radiation. Fatigue is overwhelmingly the biggest side effect I have seen in myself and my family with radiation. Everything I can find to read bears this out.
I can tell you from only my experience it took a few months to get back to really being able to work all day without hitting a wall. I had a year of surgery, chemo, and finally radiation and, it took a good year before I really felt like my energy levels were normal again.
Truth is, this feeling of energy for me was short lived as just a few years later after my first relapse, I never really felt like the energizer bunny again.
Let yourself rest when you need to, with any luck you realize one day that your energy has returned.
How many more treatment do you have to get?
Are you able to schedule the rest when you need it?