@MissDaphne - I've had mastitis and it was really dramatic! Symptoms of a clogged milk duct can include a low fever and a general feeling of unwellness. Mastitis feels more like the flu. The classic sign is a fever over 101.
You can probably give it a day or two to see if you feel better or worse. Rest as much as possible; maybe take baby to bed with you and watch some daytime TV! Nurse as often as you can, always on that side first.
They said that it's helpful to point baby's chin toward the lump or sore spot; I don't know if that's an old wives' tale or not, but it's worth trying. In any case, a clogged duct can result from always nursing in one position, so that the whole breast isn't emptied, so trying new nursing positions can help resolve the clog.
You can find other ideas online for clearing a clogged duct. Rest and feel better!