Yes, this is very common. The Hormone Therapy (not hormone replacement therapy!) tends to cause fatigue, and so does the radiotherapy, so you're getting a double whammy just at the moment. Exercise is the way to counteract the fatigue caused by hormone therapy, but you probably won't be able to start that until you've recovered from the fatigue caused by the radiotherapy unless you're already doing it.
No one is quite sure why, but one contributory factor is that both the hormone therapy and the radiotherapy cause a reduction in Hemoglobin levels, pulling you towards anemia. Additionally, the radiotherapy does damage which your body needs to heal from, and that consumes resources from your body. You should recover from the radiotherapy side of this within a couple of months.