It sounds like shell rot to me. I also have a very limited knowledge of DBT's, but it looks like shell rot and what I found when I did a little research points to shell rot. As we all know, online information needs to be taken with a grain of salt. There is a lot of good stuff out there, but there is also a lot of misinformation. The key that I saw is that it is in fresh water and more than a few sources say this is common if you don't keep it in brackish water. Chelonia.org is a great site and I trust most of what they publish. To be safe, I would contact a reptile vet or at least try treating it with betadine and drying periods. She's a gorgeous girl and i'd hate to see you lose her.
Here are a few random sites I found that may help:
http://www.diamondbackterrapin.com/index2.html
(You might want to email this guy. He looks like he runs a great operation.)
http://www.neoterrapin.com/care/
http://www.chelonia.org/articles/Malaclemyscare.htm