This might be a sign of a bleeding issue leading to fluid build up and the bruising seen.
Dogs that ingest rodent bait can have blood clotting issues which you can read about here
http://www.marvistavet.com/html/body_rat_poison.html
and dogs with tick bite disease can have a bleeging issue too
http://www.mirage-samoyeds.com/tick.htm
And this might be an issue called DIC which can be triggered with a bite
http://www.veterinary-hematology.com/DIC1104.htm
among other health issues.
Or a problem called thrombocytopenia
http://www.aquaticcommunity.com/dog/health/cardio/Thrombocytopenia.php
An allergic reaction to a bite might cause this, as could a blood clot or a problem with the lymphatic system, but if you see any abdominal bruises, red spots on gums or tongue, bleeding from nose, or anal area, or blood in urine or stool you may want the vet to check a blood coagulation time.
You can read about the lymphatic system here
http://www.peteducation.com/article.cfm?c=2+1614&aid=2975
I would suggest if your vet cannot give you a diagnosis that you consider asking for a referral to a small animal internal medicine specialist.
Hope this helps you!
Dog Expert:Rescue, Train,Breed,Care
30+ yrs dog home vet care & nursing, rescue, behavior&training, responsible show breeding, genetics