Hi Ladylove, From what I've read and spoken with a few friends about is that it is something to with the milk production, as it gets ready sometimes the tubes and things get blocked and cause the lumps.

Like Curly Girl said massage is the single best way to smooth it out. No under wires either is great.

My friend had this issue right up until after the birth of her baby, and had to sit in a warm shower and rub till they cleared (quite painful apparently) before her milk would come in properly.

Hope this helps