Chihuahuas and bad breath!

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I have a 7 year old female Chihuahua, she is healthy all over and normal. There is only one problem he breath is HORRIBLE we took her to the vet and asked about it, he said her teeth were fairly clean and she "just has bad breath". Well I have brushed her teeth a few times with mint dog toothpaste and i get plaque and her breath is better untill the next day. I have seen her eat poo a few times but I don't think thats the problem all the time. Why doe sher breath stink so bad and what are some thing you could suggest to help? Thanks!