My pug is limping his front right leg, what is it?

We cannot diagnose your dog, especially over the internet. Don't try to diagnose the dog yourself! By what you are describing though, it sounds like it's become infected, and if it is, he is definitely in pain, poor little guy! You should really take him to a vet and have him checked out. The vet is the only one who can properly diagnose and treat your dog. Hope it's nothing serious and that he's better soon!