I had it as a recurring problem for a few months with my third baby only. Not sure why... and it happened when he was a bit older (6-10 months or so). I made sure I really kept an eye on it... I did double up the feedings on each side, massaged the breast during to make sure that all areas with lumps were draining etc. And at first sign of a lump or tenderness, I'd nurse right away on that breast, massaging towards the nipple.
I feel for you - it's awfully painful... especially in those early weeks! Good for you for hanging in there!! I am still nursing my little guy now, strangely enough (he doesn't want to wean!) and he'll be 3 in May... Longest I've ever gone...
Anyhow, best of luck and keep hanging in there!