I hope your shoulder comes good, @HelenlovesSnoopy ... my suprasprinatus was involved as well! Ay yi yi! Getting older is SO much fun, eh??
Re your skin - has it actually 'errupted' into a sore, or just itchy? It can happen, apparently (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4129361/).
My husband also got very itchy skin from the cancer treatment he was on - and we found that CORNFLOUR (of all things) really helped!! I put it into an old Johnson & Johnson Baby Powder shaker - I would stand him in the shower & then I would sprinkle it all over him & 'gently rub it in'. When he got out of the shower, I'd just run the water, as it virtually dissolves immediately. It could be worth a try.
Another friend had very itchy inflamed skin & she put Uncle Toby Original Oats into a stocking & soaked it in water & it produced a milky liquid that she used instead of soap.
From the Macmillan site in the UK - they say:
This treatment can affect your skin. It may cause a rash, which may be itchy. Your doctor or nurse can tell you what to expect. If your skin feels dry, try using soap-free cleansers and unperfumed moisturising cream every day.
Always tell your doctor or nurse about any skin changes. They can give you advice and may prescribe creams or medicines to help. Changes to your skin usually improve when treatment ends.
Good luck xx