Yay! Got my drugs.
Dentist thinks the new brush head was just coincidental and that the root (no pun intended) problem is an abscess in a tooth that already had a root canal and a crown - and referred me to an endodontist. Apparently when he pushed down on my gums he saw pus come out, so definitely an infection, and x-rays indicate some break down in the socket/bone beneath.
btw, I was already rinsing with salt water.