breastmilk changes consistency during the course of a feeding session or pumping.
the milk does not get backed up nor does the fat get backed up in your milk.
some studies have shown that in winter months your body will produce milk that contains more hindmilk(the richer, fattier part of the milk.) so perhaps that is the cause. in the summer months it's believed that we produce more foremilk.
at 1:11 AM on Jan. 26, 2009