Just a thought, have you changed your shampoo, hair products, shower/skin products or clothes washing powder? I'm a few months post end of radio and my skin is still v sensitive and feels itchy sometimes. But not red like yours. I've just been using palmers coconut oil on it which seems to help (can hardly see scar now anyway).
I've not ventured back onto hair products besides conditioner for post radio skin sensitivity reasons.
Good that you have mentioned it to the onc since best to check. Hope it's calming down