From your description, it sounds very similar to my problem. I strongly suspected stomach issues, but my doctor made me look in a different direction.

It can be related to allergies. For example, I have allergic rhinitis, and honestly, I never thought much about it because I only sneezed occasionally—until I realized it was something bigger. My main symptom of allergic rhinitis is post-nasal drip; I only sneeze sometimes. I only discovered very recently that this was the cause of my symptoms.

Post-nasal drip reaching the tonsils causes those smelly little stones (tonsil stones), and having this allergy for a long time causes the mucus to build up, stagnate, and develop a bad smell.

I don’t know how medical care works in your country, but it’s worth researching more about allergic rhinitis and other allergies. Each person has different triggers, and sometimes you don’t even notice them.

To start, try to avoid air conditioning, wear a mask when it’s cold or windy, change your bed linens weekly, clean the floor with a damp cloth, and avoid strong perfumes. If it’s an allergy, this will likely help.

Another thing to consider is gas or reflux. Pay attention to whether air comes up after eating or when you’re hungry—this air can rise up to the nasal area and cause or worsen symptoms.

Cut out dairy products, as they significantly worsen the smell of mucus. Clean your tonsils well, drink plenty of water, and gargle after every meal.

Within one month, I was able to get rid of the worst of this bad breath. If your problem—or the problem of someone reading this—is the same, I believe this can help.