What you are describing at the moment with swelling on her face does sound like an allergic reaction. It can be to an insect sting, bug bite, or to a medication (is she on any at the moment?).
When I see dogs with this, I treat them with BENADRYL (diphenhydramine).The dose that one generally gives is 1mg/lb. It comes as 25 mg tablets, and if Dallas is 20lbs, you can safely give him ONE adult TABLET. This can be repeated every 8 hours for 24-36h.
Here is more about Benadryl:
You should see improvement within 30 minutes.
In the meantime, apply cool compresses (a small towel dipped in cold water and wrung out) to the affected areas.
Here is more about allergic reactions in dogs:
If the swelling is increasing in size or starting to include other areas such as around his face or nose, I suggest you take him to your veterinarian immediately. Please stay with him until it starts to come down because if it starts to cause swelling in the airways this could be serious. If it comes down with the Benadryl, it raises the index of suspicion that this is an allergic reaction.
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The above is given for information only. Although I am a licensed veterinarian, I cannot legally prescribe medicines or diagnose your pet's condition without performing a physical exam. If you have concerns about your pet I would strongly advise contacting your regular veterinarian.