Hi B.,
I had the exact same symptoms that you do now when my baby #4 was three weeks old. My OB told me that it was a clogged duct and to nurse, nurse, nurse. She also prescribed an antibiotic. Unfortunately, the bug was resistant. Within three days I became septic and had to be hospitalized. It was MRSA. Probably picked it up from the hospital but it is a community aquired infection now. I wound up having to have emergency surgery....It was horrible.
You will likely receive lots of advice about cabbage leaves (wives tales) and pointing the baby's nose toward the lump when you nurse, etc. Just PLEASE call your doctor and if you develop a fever, see a doctor ASAP.
Hope you are feeling better soon,
D.