That's probably a clogged duct or just a regular duct. Very normal!
Breast cancer lumps aren't generally painful. Ducts can be tender to the touch when they're full, and they can even be visible sometimes. Try nursing from that side to get I drained some. I had many of these when I was breastfeeding, if it get very painful and red, and you start having flu-like symptoms, get to the doc, as that's likely mastitis, and you'll need meds to treat it.
at 7:30 PM on Jan. 17, 2011