ok...I just read 7 articles about I-131 use in cats, and to date, there have been NO reported instances of cats developing cancer as a result of the treatment. The ONLY known side-effect in cats is difficulty swallowing. I think that your cat's cancer was probably already there, lurking in the background without symptoms. That's what cancer does, the sneaky, filthy disease. It lurks until it is so strong that it can't be conquered.
I am so very, very sorry for your loss.
Rest you gently, Dear Friend, dream you deep. You walk in someone's heart forever.