Thank you for providing a picture of Diesel's lump! It looks like it could be a histocytoma, which is a benign tumor, but without seeing him in person, I can't say for certain. Histocytomas typically resolve on their own in a few months, but given that it seems to be getting bigger, I would recommend having him seen by a vet on Monday for it. The vet can do a fine needle aspirate to confirm if it is a histiocytoma or not. To do this, the vet sticks a small needle into the lump to get a sample of it's cells, and examines the cells on a slide under a microscope. It's usually painless to the animal, and relatively quick to perform. If it is a histiocytoma, the vet may want to wait to see if it resolves, or can discuss removing it surgically. I hope this helps!