Lissy,
The term die-off comes from the use in candida diets where an antifungal drug,
along with diet, is used to kill an over abundance of yeast germs.  When you
kill out yeast germs your body absorbs products from these dead germs which
can temporarily worsen your symptoms.  This is my unscientific explanation.
If you have this die-off reaction it can actually be a good things because it
shows that you are getting rid of the yeast, so they say.  I have to go, but
I'll write more later about how I think that might relate to starting a milk
free diet, if you want.  Take care, Jane