The new forums count the number of PMs ( private messages ) for each user differently than the old, in that they now include sent messages in your total.
As a result, you may find that you see a message at the top right hand corner of your screen indicating that your PM box is 100% full.
To fix this, click on the 'Private Messages' link above it, then click the arrow in the drop down box entitled 'Jump to folder' ( also below ) and select 'Sent Items'.
You can empty your sent items by clicking 'Empty Folder' if you don't want to keep any of them. If you only want to delete some of your sent items, put a check in the box at the right of any messages you no longer need then at the bottom choose the 'Delete' option for selected messages and clicking the 'Go' button.