It is unlikely for SLE to be caused by the breast cancer chemotherapy. There are cases of drug-induced lupus, but this is usually when you are on the drug treatment and unlikely to occur a year after treatment. Furthermore, breast cancer chemotherapy is not a common cause of it.
As for the rash following a nerve, I agree that shingles or herpes zoster needs to considered. The zoster rash is usually associated with pain. It starts off with a red rash and blisters subsequently appear.
It is hard to be sure what the cause of your rash is, and I would suggest seeing your dermatologist/doctor to confirm the diagnosis
