Not sure what's happened... Last week I purchased some Turbine points from the store. After doing this my total character slots changed from 2 to 4. I've been using all 4 character slots ever since until I logged in tonight.
As soon as I logged in tonight and made it to the character select screen I was told I had too many characters and was prompted to select which two I wanted to keep.
One of the reasons I purchased points last week was because (from what I read) this upgraded my account and allowed me to have 4 character slots.