K.P.
My first thought was thrush as well. Check out La Leche League's website, because they have a "breastfeeding questions" area and probably address this. Even if they don't talk about this particular thing, you can look for your local LLL leader and call or email her and discuss it with her. Many times having a conversation with someone -- a back-and-forth discussion -- can rule out possibilities and pinpoint the chief suspect.
If it is thrush, make sure you treat both your breasts and the baby's mouth, so you don't pass the infection back and forth between the two of you.