Hi motton I've never heard of that happening to anyone before ... I am sure your Rad Onc will be keeping an eye on it - and noted its current size, to see if it gets any bigger. How long ago was your treatment?
Could I ask how old you are? I've got a bunch of funny things across my tummy in various sizes (that are raised & dark in colour - she gave me a name but I've forgotten it - just age related!) I've also had a large mark (thumbnail size) appear near my right eye in the last 2-3 years ... my GP says it is an 'age mark' .... it is like a gigantic freckle! No itchy or scratchy bits tho ... just weird.
She also did a full body check for cancers, so I will be starting that nasty cream soon that will turn my face beetroot red & scabby soon ... YUCK!
If you continue to be concerned, maybe get a professional 'all over body check' with an accredited skin cancer clinic? Our skin is like a blotter - it can take 'so much' before it is supersaturated & the skin then erupts! ....
I am in my 70s now - and used to get horribly sunburnt as a kid living in Qld ... and stuff is only appearing on my skin NOW!
Take care & best of luck xx