Excessive salivation, ptyalism, is normal in some animals. Some large breeds will drool constantly, and there is nothing that can be done about it. Other dogs will drool excessively around certain stimuli, for instance, food, excitement, heat. However, when this is a sudden change in the dog then you need to be worried and look for signs of something serious.
Usually, excessive drooling is caused by stomach disorders, gum infections, dental disease, injury to the tongue, insect bites, mouth ulcers, tumors, nausea and certain metabolic disorders.
You will know its something serious if there is abnormal swelling on the lips or mouth, foul odor from the mouth, inability to eat or drink, lethargy, weakness, vomiting, problems breathing.
If you notice any of these, take the dog to the vet, otherwise it is probably nothing.