Hi I finished my chemo in July and am still really fatigued from about 1pm most days, and it really surprises me some days because I feel pretty good in the morning unless I had a bad night with little sleep. Then whammo the fatigue wipes me out. I too was worrying about this, but had a good chat with my oncologist about this in mid December. She assured me that the fatigue is very common at this point in the process (so it would be true of you too), and that was such strong contender for the cause there was no reason to fear that it could be anything more untoward.
I am told this is probably mostly cognitive fatigue, and a sign that the chemo did a great job of getting into my brain to clear out nasty rogue cancer cells.So now my brain has some troubles when I work it too hard, and runs out of juice by about 1pm. The fatigue is a pain, but it is good to know that the chemo gets into all the nooks and crannies, isn't it!
Breathlessness is what I started to get before scary parts of the process. I got breathless about 2 or 3 days before my TCH chemo sessions most cycles, and often became very emotional and reactive at the same time, because I was tired and physically scared. Could that be what is happening to you because you are about to start radiotherapy?
best wishes, hope it all gets better and goes well for you