I got plugged ducts almost weekly for the first year or so of nursing. It's no fun. For me the triggers were bras, really any bra...I went without as much as possible and wore motherhood sleep bras while out of the house when I could get away with it...also sleeping on my side, and being away from baby for a couple hours. Taking lecithin regularly helped a bit. I think some people are just more susceptible to them.
Take hot showers and baths (nurse in the bath once its cooled down a bit), drink lots of water, get plenty of rest, and keep baby nursing on that side as much as possible.