Something similar happened to my dog when she was old and fragile. She had a tumor on her neck, and it began to rupture and she wouldn't eat or drink anything. She was put down. This sounds similar to my dog's situation. And since your pup seems to be older, you might have to prepare for the worst, unfortunately.
But then again, it could be a bite from an insect or something else. You really can't know until a vet gives you some sort of diagnosis. There's no spell to cure old age, especially for canines.