I agree with those above - it's probably the hormone therapy.
I had radiotherapy + Zoladex (HT) started 2.5 years ago (ish). The radiotherapy zonked me for a good six months, but but even though I knew I was past that, it left me with a tiredness, lassitude, and every other word in the thesaurus!
Over the many months, it has eased a little, but it was clear from the start that I'd have to force myself forward, or slip back. Still do. And that in itself can be exhausting some days.
Walk, be active. On days when you have nothing to do, do something. At our time of life we're like sharks; we have to keep moving.
But two small points:
1. It will probably get a little better as you progress. Keep that in mind!
2. Be self-aware: the lassitude you're likely stuck with, but be sure there's nothing else going on (eg cardiovascular, depression) which may be eased by intervention.