If you've got constipation thats most likely to be the culprit! You need to have a stool every day AT LEAST otherwise you've got food rotting inside your body and logically thats going to smell bad!
Don't bother with any expensive treatment if I were you - just clean out your body and you can do that through fibre! Cut out all Sugar and eat extremely high fibre meals - porridge for breakfast - rice/grain with dinner and lunch or something like that - lots of vegetables and fruit.
However, for a quick fix - an easyish way to stop constipation is carrots - eat like five (make sure you get enough water - but don't drink WITH food as it dilutes the stomach acid needed to break down food) or/and take linseeds which are sometimes called flaxseeds. If you have a grinder, grind them down (about 3-5 tablespoons!) mix with juice and down it, then drink lots of water to push them though...if you don't have a grinder make/buy some museli (best to go one without wheat as that can irritate bowels for some people) and add the unground shell to it (again about 2-3 spoonfuls).
If your constipation is worse then that (and I would try ALL of the above first!) theres a new laxitive out on the market that doesn't make you dependent. In England called 'Movicol'...basically its a salt mixture that prevents the water you take with it from being absorbed into the body so it goes from your mouth to your exit...its basically the new colonic irrigation but from the top down rather than bottom up.
Remember that your body isn't compartmentalised - and mostly one symptom is part of a different problem. Also - bad breath is NOT a disease but a symptom - so it can be cured when you work out what it is.
If your constipated - its most likely to be that - and once you get your bowels moving to one-three stools a day - you should be fine! Good luck and let us know how your getting one!