I'd bring it to a vet to be safe. It could be so many things. An allergic reaction to anything, canine acne, start of an infection from bacteria. Anything! Whatever you do, DONT pop it! Don't touch it with dirty hands either.
That's all good advice from Finn. Be sure to be able to accurately describe what you are seeing in case it changes before you can get your dog examined.
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Posted by Cindy