I had dental problems after having babies. I didn't get cavities, but I had gum issues. I've found out after research that not only is it calcium loss during pregnancy and nursing, but the hormones of both cause your gums to swell and or recede. This causes cavities. I was lucky to avoid any cavities, but I've had to have some extra gum work done over the years ($$) and be super diligent about flossing. My gum problems are over now, but they lasted for years after my last child was born.
We've had issues with our dentist and we don't trust her at all. If she said I had 13 cavities, I'd be getting a second opinion too. But, there is reasoning behind why you'd have cavities and dental problems after having so many children in succession. I hope your second opinion is a better one.