A rash is not good - ring your Breast Care nurse or the office or hospital where your surgery was. It maybe you have something as simple as an allergic reaction to a cream or soap you are using as the skin and tissue go through the recovery process. Don't sit back and wait for your GP to return. Ring for an appointment with an alternate GP if you can't get onto the breast care nurse.
I had an angry rash that spread quickly and put me in hospital for 5 days of intense antibiotics and further antibiotics for 6 weeks. Mine was hot but not itchy
Take care