This looks like it could be a benign tumor like a wart, but without seeing Tilly in person, I can't say for certain. 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. I would give her vet a call in the morning to make her an appointment. Best of luck.