Hello, you are not alone in this. I find out next week if I am still in remission after 1 year since treatment. Mine was lung cancer with chemo and radiation therapy, no surgery, but other chronic illness to boot beside COPD. Treatments can effect each of us in different ways. May I suggest that you talk to your Radiotherapist and your doctor that helps with the COPD, maybe even get them to chat with one another in reference to your problem.
Knowing you are going through such a whirl wind of things right now there could be other things to bring to their attention and even yours. Pain, are you having any pain that could be causing you to breath a little harder, for pain can get the heart racing which will cause the lungs to work harder. Are you worried about thing all the time, like what's next? I sure was and at times still am. Feeling like crud, radiation can do that to you. All these things and others can make the COPD miserable. Please chat about all this stuff with your doctors, physical and emotional there is ways they can help, could be time for a change in your COPD meds also.
By what you have already been through, you are a strong person, you can fight this and win!
Please keep us up to date on how you are doing. Big hugs, doris