Has your dog had any vaccinations or shots of any kind recently? Sometimes a smaller lump will form at a vaccination site - but it usually forms within a day or two after the shot. It could also be something called a fatty tumour which is relatively harmless as long as it doesn't hamper movement. But, alas, there is no sure-fire way to tell without pathologies. Which can be expensive. You can take it to your veterinarian and see what they think (recommended), although they may do nothing but give some anti-biotic and steroids and tell you to watch it.