If your cat is eating and drinking normally, urinating and defecting normally and is acting normally it may be nothing to worry about.
It could be due to an insect bite, an ear infection, or a cyst or other dermal lesion. Does your cat scratch his ears or the lesion? Ear infections can cause a cat to scratch their ears and cause dermal lesions.
If you are really worried about your cat, a visit to your vet may be a good idea. Your vet can look into your cats ear or take a needle biopsy of the lesion for analysis.
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