With the limited history and view of the photo, it is likely to be shingles. Shingles can be pain free in an elderly.
The other possibility is skin vasculitis (inflammation of blood vessels in the skin).
I would advise you to send her to see a doctor as soon as you can for a proper assessment.
Hope she gets well soon.
Regards,
Dr Liew Hui Min
Consultant Dermatologist
