How perceptive of you Mr A...
Seriously though, I'm not 100% sure why this is happening - or rather I know *why* it happens, but *not* why it's happening specifically to you.
Try clearing both your cookies and browser cache then log in and log out by way of the "log out" button. That should reset your session and cure the issue.
If it doesn't, let us know. Be helpful if anyone else who is having this issue lets us know.
The alternative is that FD may be working on the DB and has been clearing the session table
JF