I know the women at DDs daycare always would tell me when DD1 was getting an ear infection, and they knew because they smelled her breath. These are old South American women who know their sh*t when it comes to kids, so I would go with some sort of ENT issue that may be chronic, or LO has had is off and on, and you may not remember her breath not smelling for a little while in between episodes.
that is weird! Never heard that before. I wonder! She doesn't seem overly fussy and now the breath is fine. As soon as I smell it getting bad again I'll have to watch her.