With a first pregnancy, the reason your breasts swell, is that you are developing milk ducts and reservoirs. Generally with a first pregnancy you do not produce colostrum until the last trimester, because your ducts have not fully developed yet.
With any subsequent pregnancy, you may begin producing milk at any time- and may not have completely stopped lactating from the prior birth (depending on the age of the child- In the 9 years since my dd was born, I NEVER stopped lactating between pregnancies, and I STILL AM).
It may very well be a plugged duct, especially if you wear an ill fitting bra (You should be fitted by someone who fits ONLY maternity/nursing bras, to ensure it fits properly for milk production).
Use a warm compress, take some tylenol, and massage it. You may also try to express by hand a small amount (It will not trigger labor, but may cause some BH contractions).
at 11:36 AM on Jun. 28, 2010