I had that last night and previously as well.
It seems to me that if you are using the website and leave it running but don't actually use it for awhile, it goes dormant and forgets that you are logged in.
I often look at the forum from my phone or Kindle tablet and if I don't specifically close the browser when I am done this will sometimes happen to me.
I had to close my browser and then re-open it and log in again for that to go away.
Once I did that, everything seemed to be ok.