Just noticed my babe doing it too ( one side scratching until bleeding for about a month age 6 months) but upon closer inspection he has a super tight muscle running from his neck to shoulder so so have signed him up for a physio assessment. I’m guessing he may have a shoulder injury due to his birth ( big baby and challenging getting him out) but we will find out more. Feels relieving to know he was trying to tell me something. He also has mild excema so that’s what I originally thought it was but feeling around it is obvious something is going on that is muscular.