Just wanted to tell of my experience with plugged ducts/mastitis while
nursing my son.  It was continuous for 9 months - I never stopped getting
them.  It was extremely painful(every day), and I, too, would pass stringy
thick clots at times.  Nobody every figured out what the cause was - one of
my "theories" was extreme stress.  Would this Mom happen to be under alot of
stressors?  Antibiotics would help briefly(very briefly).  I never had any
problems when nursing my daughter(who is older).  One of the other theories I
had, was that maybe I had yeast towards the beginning that entered my breast
through a crack, etc and I never was treated for yeast(centrally - only
topically).  I hope that helps some.
  Sincerely, Linda Marie Bradley, RN,BSN,IBCLC