it does sound like a seizure, but the dog isn't incontinent which is one of the surest signs. Hard to tell without another diagnosis. If it is seizure activity, it could be caused by low blood sugar, don't know if your vet checked this or not. A complete blood profile would be in order I would think.
Also, bee or wasp stings can cause this for long periods, especially the salivating. The stinger could have gotten lodged in the nose and caused a small cyst or in the throat also.
In any case I would have the dog checked out again.
Is there any way the dog could have gotten into some kind of insecticide? This will also cause similar symptoms.