Angella Michells I am not a vet, and don't offer medical advice, but there are a lot of things it could be, including cancer. How old is the dog? Is the lump really hard or alittle squishy? Many dogs as they get older, develope fatty tumors, they usually are softer and where theres alot of fat anyway.like on their chest and,stomach, If it is in a joint, does it seem to bother the dogs movement? It is always best bring the dog to a vet to have it checked out. I wish you and your pet the best of luck!