I may be adding 1 + 1 and coming up with 11, but just a thought.
I've been doing Oil Pulling for about two years. One of the things that happened when I started out was the Gums receeded. The common consensus is it's caused by the toxins being pulled from the Gums by the oil. Even though the gums may have looked healthy, they were swollen from the toxins. Over time my Gums have come back, but not to where they were. The relationship being that you may be purging toxins from your body on the MC...including the Gums, causing them to receed.
I have not had any experiences with bleeding Gums.