Thanks for posting your query here.
Does your granddaughter have any fever? Is she able to take fluids/feeds well? You've not mentioned about her age.
It is difficult to diagnose without seeing, but one of the possibility is infection - spreading infection of skin called cellulitis
in medical terms.
If she is unwell, running fever and the rash
is spreading such fast - it would be advisable to see Pediatric doctor.
Are there any other symptoms? Is the weeping wound from skin looking like pus?
Please revert back with information I would like to know.
Dr Tejas Shah