Z.A.
depends:
- dehydration (number one cause for bad/funny breath)
- pustules (the white things that form in the back of the throat ... a sign of strep)
- the infected mucus (each bacterial/viral respiratory infection actually has a unique smell that goes along with it... a certain kind of pneumonia actually smells like peanut butter)
- the medication smell mixed with the mucosal smell
- certain poisons give off distinctive odors
- nursing babies all have "butter breath"
Tons of reasons. For a definitive answer... pediatrician time. You can even call the nurses line at any hospital 24/7 describe the issue, and they'll be able to ask you questions and determine whether to bring in immediately, wait until morning, or not to worry at all (that one would come with an explanation).