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There is a couple of things that can cause this. The most common would be an insect bite or sting. The other possibility might be a tooth issue. With an allergic reaction you do swelling like you describe and in most cases you never see the actual bite or sting especially if they tried to eat or ingest the insect. Try some benadryl. The dosage is up to 2mg per pound every 8 hours. This should take care of the problem.
Examine the dogs mouth looking for broken teeth or discolored teeth that might indicate infection. You can't just judge by swelling since an allergic reaction would cause swelling as well. If your dog is indeed a puppy it is less likely to be a tooth abscess. Let me give you a site on insect bites and another one for tooth abscesses.
If it is a tooth issue, you will have to have her seen by your vet so appropriate treatment can be started.
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