S.T.
Is there a white film around the canker sores? If so then it's thrush - my first child had it. It's a mild fungal thing that gets passed back and forth between baby's mouth and your nipple. It is somewhat painful apparently. The pediatrician gave us an anti-fungal and it went away pretty quickly. I had to swab my nipples after nursing too. It's not a big deal at all. My "baby" is now in high school and no worse for the thrust. ;o)