You should take your dog to the vet if the lump does not go away after a day. The dog could have been injured or gotten bit by an insect or an animal. Your vet will examine the dog and tell you what the best treatment plan would be best.
If the lump on the outside of your Shih Tzu's lip does not seem to be bothering your dog for example he is not scratching at it or it is effecting his eating then just keep on eye on it. If the lump persists you should consult your veterinarian.