Contact La Leche League. They are a wonderful resource for breastfeeding moms. I've called them quite a few times from nursing strikes to mastitis and they were a great help.
Just from my own experience, if you are wearing constrictive clothing (bra, shirt, etc.) that can put pressure on your breast(s) causing soreness and/or a plugged duct. Make sure your bra is roomy as well as your clothes in the chest area. Having your little one nurse as much as possible on the sore breast is great, but also apply a heat compress (or warm washcloth) before nursing, and get extra rest. You can also massage your breast downwards while nursing or after applying heat to try to loosen the plugged duct and get it moving out of your breast.
Hope this helps (always works for me)!