Everyone will have the same problem at the moment, there's a bug in the forum. What you have to do is go to messages and click on Empty PM folders and check the bottom box only, that will empty your sent items folder.
Liz has told Forumer and they are trying to fix the problem!
I forgot to say you will either have to delete all your sent messages or none of them. That annoyed me because I wanted to keep some but couldn't!