The problem would not be from using the peroxide as a spritz in the mouth, generally speaking !
But, it could be from a combination of things, including the ACV, if more was taken than what the foods you are eating have enough extra Alkaline Minerals in them to react against this extra ACV !
Another question I am going to need to ask, is are you sure the ACV you are using is Pure ACV, or is it possible it is some of this stuff in the stores they now try to pass off as ACV, but it is really Grain Vinegar and listed either as artifical or Flavored ACV ?
Also are you supplying your Digestive system any Good Quality Probiotics with Acidophilus and Bifidus in it, to help extablish the good Digestive Bacteria that is needed for proper Digestion and Bowel movements ?