It does sounds like mastitis. Mastitis is an inflammation of the breast that could be caused by obstruction, infection or an allergy. It may come on abruptly and is usually and is usually associated with fever and likely to be more painful than a blocked duct.
Try using heat and gentle massage before nursing may help ease the discomfort. While nursing, continue to nurse on the affected breast and try to get a good latch. After nursing, pump or express milk to help empty the breast. Apply cold compress to ease pain and inflammation.
For more information on both conditions, you may find this article useful:
http://kellymom.com/bf/concerns/mother/mastitis/
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