Colgate provides an explanation of stomach problems that can cause bad breath. Livestrong.com and Healthline.com also describe the possible reasons for bad breath including stomach problems.
Colgate explains that two possible causes for bad breath from stomach gas are acid reflux or GERD and stomach ulcers.
HuffingtonPost.com also offers a list of five possible stomach-related causes of bad breath. These include eating odorous foods, possible bowel obstructions and "leaky gut" syndrome, which is a blanket term for unidentified stomach issues.
LiveScience.com provides a list of five ways to get rid of bad breath such as brushing teeth, checking for stomach problems and changing one's diet.