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Licking the bottom usually means that the dog has a problem with his anal glands. The glands normally secrete a substance every time your dog poops. Sometimes this fails to happen and the glands get overly full or impacted. If you suspect this, you can see your vet or groomer to express the glands.
The chin scratching COULD be an allergy. I suggest that you keep an eye on it and take preventative measures if it does not clear up on its own soon. This might include using an antihistamine or steroids to control the itch.
Let me know if you have more questions about this.