If there are no mites or lice (which wouldn't be in the ear, but in the fur around the sore), the most likely cause of a sore like this is a fungus. Such fungal infections can take on many different appearances. The link I'll give you in a moment has photos of some of them.
The best thing you can do is take her to a vet for a accurate diagnosis and treatment. However, you can try a home treatment first if you want to. Home treatment involves the use of over-the-counter medicated shampoos and antifungals such as Monistat 7 or Lotrimin. Here is a link that will give you all the details:
If you decide to take this route, be sure to follow the instructions exactly. Some products can be toxic if used incorrectly. No chemical is 100% safe. If you don't see improvement within a week or so, you'll need to see a vet.
The other explanation for this sore is that it could be a wound. This is more likely if you have more than one Guinea pig in the cage. If she is your only one, you probably would have seen an injury.
Again, my first recommendation is to see a vet, but of course, you'll have to decide how comfortable you are with trying to treat this yourself.
If you have more questions about this, just let me know. I hope it clears up quickly.
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