Call your cancer center and tell your nurses or the doctor, for breathing problems. That's a good question for them, because you have so much going on. If you're having extreme breathing problems, I would treat it as an emergency.
It wouldn't be unusual with that much going on, to be a little out of breath. But it's not possible for anyone to evaluate your individual case on the internet. So I would check with your nurses at least to see what they think. My dad hasn't received radiation, but he's been through two different chemotherapies. He has some breathing problems anyway from his other health conditions. But I did notice additional difficulty from the chemo. I am guessing this was from lower red blood counts. Your red blood cells are the little trucks that carry oxygen around your body.