My little Rat Terrier's mouth is swollen?

This morning I noticed my dog's muzzle was swollen on the outside of his mouth.It certainly wasn't there last night. Anyhow, he is still himself and perky like normal. I gently rubbed it to see if it was causing any pain, but he didn't mind and it appears to be having no impression on him at all. Last night him and my other dog were play fighting pretty rough, and so I'm wondering if he might have bumped himself pretty good, but like I said he is showing no pain, and he is not pawing at it. So do you have any ideas what this might be? How can I help it to go down? Any answers are appreciated. :)