It could be a bad tooth or bad filling. The stomach cleanses the body when allowed to and bad breathe can be the smell of the waste that enters the stomach via the blood or even a ulcer.
Everyone who ever fast to restore their health experiences bad breathe as well.
The cure will always to restore the health via proper foods. In the mean time you can use a little pepperment, etc.
The use of all green foods and grasses will be of much aid.