I'm not sure. I've never had that issue, and all told I've nursed my kids for a combined total of almost 4 years so far. It does sound like it's your ducts, otherwise the lumps wouldn't get bigger or smaller depending on how much milk you have at the moment.
You could read this article from kellymom http://kellymom.com/bf/concerns/mother/recurrent-mastitis/ and see if any of the advice there sounds helpful. Though, if it's just lumps and they never get sore or hot, and they go away after a feeding, I wouldn't worry too much about it.