Does my dog have an abscessed tooth or could it be an allergic reaction?

First off, I just want to say that the plan is to take him into the vet tomorrow to have him checked out.

He’s an 11 year old Chihuahua/Pomeranian and I noticed a few days ago that there was swelling on his left cheek and his upper gums/left side. I’m thinking that, maybe, it’s an abscessed tooth. The thing is, he was eating his hard food just fine (although, I went ahead and softened his food because I was worried that he was in pain). When I got up this morning, I had noticed that the swelling had gone down quite a bit on his cheek and in the inside of his mouth.

Not sure if it was an allergic reaction to something or that something is wrong with his tooth. The last few days he was pretty lethargic, but today he is back to his old, bouncy self.

Has anyone ever gone through this with one of their dogs. What do you all think that it is?