Possibly both, which doesn't help much. I haven't had radiation so haven't any personal experience, although fatigue seems very common. Working through isn't the same for everyone - even if you are physically OK to do so, some find it a great distraction from obsessing about cancer/treatment while other find it an additional burden, irrespective of their normal feelings about their day job. Pushing it may not be helpful - possibly taking a break, and giving your mind some rest, if that's possible, may help kick start your concentration again. Best wishes.