Hi George.
I highly doubt that the rads had anything to do with your cold. More likely, you can get a exposed to a cold just going back and forth to a medical facility where there are lots of people and germs.
As we all emphasize, every patients experience is unique, but my first few weeks of rads were so side effect free, I was fooled into thinking that somehow it wasn't affecting me as much as other people.
Now, on the chemo aspect, that brings your resistance to germs down, but even that takes a bit, I think.
But certainly, going forward, be careful to keep your hands clean, and try to minimize crowds where you may pick up germs.
My insurance company has a nurse overseeing my medical history, and she suggested during and after my chemo, to wear a face mask going in public.
I was embarrassed to do that, so I limited my encounters in public spaces.
Best wishes.