A plugged or clogged milk duct usually feels like a hard, tender swelling in the breast which can vary in size from a pea to a peach and may feel painful while breastfeeding. Plugged ducts occur when milk flow is restricted, or there is a delay in removal of milk, resulting in poor drainage of the breast.  An untreated plugged or clogged duct can quickly lead to a painful bacterial infection in the breast called mastitis. The latest updated tips below will hopefully help you clear the plug and avoid the onset of mastitis.