If you purchased a puppy at age 8-12 weeks, and all of a sudden it starts to develop Bad Breath (Halitosis), don't be terribly alarmed as it is quite common.
Puppies can develop a fowl smelling breath as they teeth, and most likely it will go away after the full set of adult teeth are grown in (5-7 months).
You might have the vet check your puppy for impacted teeth,or any other underlying issues, but otherwise, you just have to put up with the odor for a little while.