What causes bad breath even after maintaining good oral hygiene?

Hello doctor,

I have an extremely embarrassing issue with bad breath. It is not your typical "bad breath". It smells like dog poop, I am so embarrassed to say that, but it is true. It smells terrible even after I brush. It has affected me my whole life. Over the years I have had people consistently make comments about how bad my breath is. I have great dental hygiene; see the dentist twice a year, brush twice a day, floss throughout the day, mouthwash, etc., and nothing helps.

Finally now that I am a 38-year-old woman I mustered up the courage to talk to my dentist about it and also to see an ENT doctor about it and they both have stated they do not believe it is a mouth or ear/nose/throat issue. Probably about 15 years ago I was on an antibiotic for something and it amazingly took my bad breath breath away and when I had a bowel movement the smell was not as bad either. That is also an odor issue I have, I am extremely embarrassed about my bathroom smells, I have IBS-D so it is a common thing in my life. I remember when I took this antibiotic noticing that it "took away" my bad smells. I had typical "not so pleasant" odors but nothing extreme and I could also tell in my mouth I did not have this kind of yucky taste that I commonly have. I was so relieved, but that was short lived once the antibiotics were gone. I cannot remember at all what the antibiotic was (maybe Doxycycline or Metronidazole). I am desperate for advise or help. This has effected me in so many ways in my life. I try not to get close to people when I talk and I avoid my husband like the plague when he tries to get close, I am desperate to find some help with this issue.