the thing is with dogs is they eat, all sorts of treats and different types of dog food that is in meat form or dried form, and like us a mixture of foods can send you smelling both ends, usually its a change that does it or just feeding him or her different treats or even human foods but after saying that, it is best to just have him checked as an experts advise is always best, its probaly nothing mate, and i would say that if you licked your bum as much as that your breath would smell lol, no but on a serious note have him checked by a vet.