S.,
I too agree that you should talk to a pediatric dentist. HOWEVER, be sure that you take the time to seek out a breastfeeding friendly dentist. Many pediatric dentists do not understand the difference between breastmilk and formula, and they treat breastfed babies the same as formula fed babies - they will recommend nightweaning because bottles at night can cause decay or they will recommend weaning completely because they think breastmilk poses some kind of "danger" to your baby's teeth.
As you well know, breastmilk and formula are completely different, and their effects on your baby's teeth are also quite different. If you have a local La Leche League group, they might have some recommendations for breastfeeding friendly dentists. You can find a local LLL group at http://www.lllusa.org/groups.php. You could also post on this forum to see if any are in your area that are recommended by other nursing moms.
Definitely see a dentist, but realize that despite extensive schooling in dentistry, they often know next to nothing about breastfeeding or breastmilk. Unfortunately, not many medical professionals do. There is no money in breastmilk, since it can't be marketed, so there is very little research about it. I would be extremely cautious about any dentist who advises you to wean your child.
Best of luck,
S.