I looked it up, and all it says for most places is that it could be ammonia poisoning... But, considering you've done pretty good for your betta under that notion, I want to aim that it may be a parasite.... But I am not sure. can you get a magnifying glass? and possibly a light? Try putting the light above the betta in low water, use the magnifying glass to examine the betta thoroughly. Look for discoloration, powder look (velvet, ich), blackened/reddened gills, curled fins, fuzzy spots... check also the scales - raised, or not.
are the red spots actual spots or areas?
are her fins clamped?
any other signs you get from the examination mention here and maybe we can figure it out with you!
are you treating her in the 20 gallon with the others? or in a 1 gallon? floating a 1 gallon container in her 20 gallon will not only stpo anything contagious from spreading, but then is easier to do medications. this way needs every day water change.