I am totall convinced that too tight or just not fitting bras cause
plugged ducts.  With my second child I had a chronic problem
until I figured out that the plugged duct always came the
day after I had worn a certain bra.  Over the years I have
often encouraged women to take their bras off (many still
think that you musst wear a bra 24 hours) particularly if the breast/s
are sore.  It generally seems to work.

Sarah Barnett