Dog having trouble closing mouth?

My 3 year old beagle is having trouble closing his mouth. It is kind of like his jaw locks when he is trying to close it. We went to the vet this morning and he just thought he was having trouble due to inflammation with his allergies. He takes medication twice a day for allergies. We were send home with a medicine for inflammation and nothing else. He is very lethargic and has one runny eye. He was not running a fever as of this morning. Am I just being paranoid or should I get a second opinion? Any ideas of what it could be other than allergies. I know I am not giving the medicine a chance but my dogs are members of our family and I would hate for this to be something worse. When my mastiff was sick last year we went to the vet three separate times and ended up going to a new vet before they figured out what was actually wrong. If anyone thinks they might know what it is or they have had this experience please let me know.