The lump is about 4" wide, and painful. I'm traveling, away from home, and don't feel it appropriate to go to an ER since it's not life or death. But I don't know what else to do except wait to get home. Please send me any information you can.
This sounds very much like a large breast boil. As you can see by reading this conversation about breast boils, they're not unusual; a lot of women have to deal with them on a regular basis. Our breast boil FAQS offer lots of good information about what causes these, and what you can do about them.
If it's not a boil, it's clearly some kind of major infection. Either way, I think it's Top of FormBottom of Form, entirely appropriate to go to the ER for this. The last thing you want to do is wait until you get home to get it examined and treated. You might try going to the hospital, and asking if they have a walk-in clinic, if you'd prefer not to go to the ER for insurance reasons. But you should DEFINITELY see a doctor about this ASAP, OK?