My first guess is the minty freshness, rather, the fact that the common flavorings are nerve excitants and are supposed to provide "hot" stimulation. Unfortunately, the mint, peppermint, wintergreen and other "essential oils" can burn like hell when they come into contact with sensitive areas and around 1000x worse if they creep inside to sensitive mucus membranes. I have had that happen to me with a surprise peppermint oil BBBJ. I had to dash for the shower and spend several minutes (seemed like 20 minutes) trying to put out the fire.  
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The texture could be interesting, too.  
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Maybe she was an efficiency expert trying to brush her teeth AND give you a BBBJ at the same time? You know, like answering text messages while getting boinked doggy style.