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This is often caused by an insect bite or a bee sting. I would pay close attention to it and see what it does. If it seems to be growing or she seems to have trouble breathing or is swelling, your vet might want you to try a dose of benadryl. An antihistamine can give her comfort and reduce swelling. The dosage that my vet recommends is 1 mg per lb of dog given every 8-12 hours as needed.
Let me know if you still have questions.