Might be a lymph node swelling up.
Or if you live where insects are still active could be an insect sting or bite.
Or if he has been playing with another dog might be an abcess from a small tooth hole that sealed over and grew infection under it. It would seem sore if that was the case.
He might have popped a blood vessel too causing a big hematoma.
As it has come up so quickly you really should have the vet check it out. If its a lymph node you will want to find out what is triggering that swelling as it could be something such as a tick carried disease that would do better with quick treatment.
Hope this helps you!
Edited by NHolmes on December 21 2005 at 5:32pm
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