Get your dog in a corner of the sofa or somewhere else where you can control it, with a bright light to help you see in the mouth. Now check for foreign matter - usually twigs, but it could be anything. You may find a bit of twig, bone or whatever jammed up into the gum between two teeth. Or, and I've read about this, there may actually be a piece of something jammed into the soft palate. Good luck.
Has there been some kind of injury you know about, or accident?
What do you mean by "hurt?" Is he bleeeding? Unable to eat or drink? Shaking his head or having problems breathing?