I don't know the Shitzu breed well but I can say it shouldn't have any lumps anywhere. So what is it? What does it look like, what size, can you see it or did you discover it because while petting you felt it with your hand? Is this new? Are there any unusual behaviors by the dog?
I don't run to the vet with every little problem, so assuming the condition looks minor I would give it a day or two to see what develops, keep a close eye/hand on the dog. If it gets worse, I'd go to the vet, if it is awful now, I go to the vet now.