You should go to your GP and get blood tests done, Full Blood Count (FBC), Vitamin B12, iron.
Both the hormone therapy and the radiotherapy reduce your hemoglobin levels and this double whammy might have left you anemic, although you should be recovering from the radiotherapy effect by now. It might be that if you are short of B12 or iron, recovery could be slower, hence checking the levels.
Breathlessness can be caused by HT, but also might be made temporarily worse by the radiotherapy.