What is this growth under my dog's mouth? Should I see a vet?

Thank you for providing pictures! It looks like Sophie could have a growth of some sort there, either benign or malignant. Any suspicious lumps should be examined by the vet. The vet can perform a test called a fine needle aspirate on it. To do this, the vet sticks a small needle into the lump to get a sample of its cells, and examines the cells on a slide under a microscope. It's usually painless to the animal, and relatively quick to perform. Once the cause of the lump is diagnosed, the vet can discuss treatment options if needed. Give your vet a call in the morning to make her an appointment. Best of luck, I hope this helps!