It sounds to me like it might be a clogged duct...
Nurse frequently, and while you nurse try to "massage" your breasts in circles (like if you were looking for lumps in a breast exam)... I had a few times that I got temporarily clogged ducts and my breast would be so tender and painful and the suddenly, usually during nursing... i would feel a "click" and the milk would release. massage all the way up to your armpit, down to underneath your breast... your breast tissue can actually cover a lot more area than you would think...
If it hurts too bad to nurse, or you feel sick or feverish... go to the doctor in case you are developing mastatis!
if the baby is not wanting to nurse right now,, try pumping or even hand expressing... until it starts to feel less uncomfortable-
Good Luck!
-M.
PS... make sure the baby has a good, wide latch too... and isn't actually nursing on your nipple itself... that can make you really sore!