Take a good look inside of your dog’s mouth. See if you notice any teeth that could be broken or damaged. I had a similar experience with my dog and I noticed he had broken his front canine tooth in half. The pain reflex caused heavy drooling but as soon as it was removed he was fine. If all of the teeth look okay you might just want to give him a once over and see if you might notice any bumps or bites or rashes that could be causing the problem. Either way a trip to the vet is probably needed before the problem gets any worse.
My sister in law has a germanshepherd mix and he was shivering and drooling out of his mouth and nose we looked him over and found that he had a bone that she had given him the day before lodged between the back of his mouth and into his throut it was stuck on his back teeth I had to jerk it off of his teeth and then he was fine again do you give your dog bones and if so check the inside of the mouth