Is this a plugged duct?
A plugged duct is a tender, hard lump that suddenly develops in one breast. This happens when milk is unable to get out of the breast with breastfeeding or expressing and causes pain and swelling. The lump is generally round with regular edges that are easily felt and between the size of a marble and one-third of the breast. It may develop along with a milk pimple. The plug may clear within a day or so after forming if the milk can leave the breast. If not, the body will dissolve the milk but this can take several weeks. Having a plugged duct can increase the risk of a reduced milk supply, mastitis, or an abscess.