How big is it. It could be a pimple or a cyst or a fatty tissue deposit. If he was just at the vet last week and the vet saw nothing, my guess it is minor. Buster (my RB lab) had fatty deposits for 5 years before he passed on and when he did it had nothing to do with his "lumps". They looked like big lumps - but were harmless. Call the vet on Monday and talk to him/her. Cancerous lumps do not appear overnight. If you are really anxious - try the websites that offer "Ask the Vet".