Thank you for contacting Petco Pet Education Center, formerly Petcoach with your question about Princess and for providing pictures, they are very helpful. Yes, this lump may be concerning as she is a senior dog and there is a chance it could be 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 her vet a call in the morning to get her an appointment. I hope this helps!