There are some things I'd suspect with this. One is possibly a pancreatic episode. See if you can track this to a food treat given or something like a heartworm chewable treat being given.
Yes bloodwork should have shown something but it doesn't always.
Another is something seen more in smaller dogs called
http://www.sniksnak.com/doghealth/hge.html
Salmonella and Campylobacter bacteria, like E.coli and other bacterial species like giardia can cause this too.
Food allergies can cause this. A food treat, a heartworm chewable with beef in it when the dog is intolerant of beef, anything like that can trigger this sort of reaction.
If your vet has no ideas you might want to see a veterinarian who specializes in small animal internal medicine. This site may help you find one
http://www.acvim.org/Specialist/Search.aspx
Hope this helps you!
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