Hi Sharon - sorry to hear about the itchy hell you're going through, it would sound as though the CS shakes are triggering it, if its happening soon after drinking one.
I know how terrible it is, after my daughter was born and my milk came in, I was covered in a rash which was itchy as hell, the community nurse said I'd taken a reaction to my milk, which is uncommon, and it would take a week or so to go away and to take an antihistermine to reduce the itch - it was hell but did go away within a week, but I never had this with my previous child and I'd never heard of it before!
Hope it gets better soon and if it is the CS shakes, could you try another brand of shake (Tesco even do their own now) with similar cals and carbs and following the CS rules? Take care hun x