Kitten has a infection on his upper lip

Hi i recently noticed this one again the last time he had this infection on the other side of his bottom lip the vets gave me antibiotics to treat it but its happened once again what do you think could have caused this?

It could be a rodent ulcer, which has nothing to do with rodents.  It's clinical name is Eosinophilic granuloma and is usually an allergic reaction.  You can try changing to a grain-free diet and see if that helps. Also, keep your home as stress-free as possible.