I tried to search online to see if others have experienced the same where oil pulling spreads cavities, but almost all references I found are saying it is supposed to help fight tooth decay. Not sure why it did not work for you and actually caused deterioration. I'm curious what oil you used in your practice.
Here is Dr. Mercola discusses "How Coconut Oil Might Combat Tooth Decay ".
Coconut Oil Might Combat Tooth Decay - Mercola
And here is someone sharing her own experience healing tooth cavities with oil pooling:
Healing Cavities (A True "We've-Done-It!" Story!) - Trina Holden
P.S. I myself have not tried oil pulling long enough and consistently enough to notice any effect one way or another.