Could the dog have been bitten by something like a bee or a spider?
This swelling could also be a circulatory problem. When the fluid in the tissues cannot be picked up and returned to the blood circulation, there will be edema. There may be a blood clot somewhere or there may be an enlarged lymph node preventinglymphatic drainage. You should let your vet examine the dog and possibly give some Lasix to help remove the fluid.
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Dr. Louis Gotthelf
Animal Hospital of Montgomery
Pet Skin and Ear Clinic
