Assuming yeast, etc. has been ruled out,  I have had some success having the
mother nurse "upside-down."  Well, not totally.  Mom is semi-reclining and the
baby is face down at about a 45 degree (less or more) angle and thereby
grasping the aerola at 180 degree opposite where usual grasped when "right-
side-up".  It especially works well for an older baby. I believe Jack's babies
were 7 months old.
Might be worth a try.  (Can be a problem if baby spits up a lot!!!!)
Linda Pincus, RN, IBCLC