This may be due to infrequent breastfeeding, poor attachment, tight clothing or trauma to the breast.

Sometimes the duct to one part of the breast is blocked by thickened milk.

[1] A blocked milk duct has the following common symptoms:[2][3] Blocked milk ducts are a common breastfeeding problem and can be caused due to a number of reasons:[2][4] The most effective treatment against blocked milk ducts is to empty the affected breasts by frequent breastfeeding or pumping.

[5] Numerous other treatment approaches have been suggested, however, there is insufficient clinical research to determine the effectiveness.

Treatments that have been studied but have no strong evidence for or against their use: A blocked milk duct can result from a nipple bleb.