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Blocked or plugged ducts are a common source of pain when breastfeeding. When a milk duct becomes blocked, it can become tender and inflamed. Unlike mastitis, plugged ducts are not associated with fever and often get better if you breastfeed more often. Other treatments might include massaging your breast and applying warm compresses to the area.
Mastitis causes symptoms similar to a blocked duct, but you will likely also have a fever and other flu-like symptoms. While a plugged duct is caused by milk stasis (milk sitting in the breast too long), mastitis is actually an infection. Treatments are similar, though, including breastfeeding more on the side that is affected. You may need to take an antibiotic if you aren't quickly feeling better.