If you have any type of nipple or breast pain, get help from a board certified lactation consultant as soon as possible. The longer you wait, the worse your problem can become. "Milk blisters" are small painful blisters on the nipple. They are sometimes connected to a plugged milk duct.
From the editors: In the meantime, try putting a hot wet compress on the blister before nursing or pumping. Then, if there's still a plug or skin over the duct, rub it with a washcloth and try gentling scraping with clean fingernails. Start nursing on this side. Also, if milk blisters are a recurring problem, talk to your doctor about a lecithin supplement.