Lisa:  I don't know what causes this, but I had a patient who nursed
primarily from one side because the baby preferred it, and the other side
never produced as much milk.  The side which she did nurse on was
chronically mildly red and vascular in appearance, mostly on the
underside, but never painfully firm or engorged, there was never a mass
(I even sent her to a breast surgeon for an opinion, as I didn't want to
miss an inflammatory cancer).  After she weaned, it went back to normal.
Alicia.  dermer[umdnj.edu