The first step would be to get the condition diagnosed by a veterinarian that has examined the dog. You can’t treat something if you don’t know what it is.
In the meantime, if it is not an emergency and you have to wait a day or two for a vet appointment, you should bathe her with a gentle shampoo and see if that helps.
If she has a bright red rash all over her chest area and /or is experiencing significant discomfort, I would consider it an emergency and go to the nearest 24 hour veterinary clinic.
If her regular vet has an answering service that will page the on-call veterinarian, you could try that, they will advise you, maybe they can fit her in for an appointment first thing in the morning.
