Being automatically logged off during posting.

I'm not sure where this post belongs so please forgive if it is out of it's place. Why do I get automatically logged off, sometimes, after posting? it happened earlier today. After finishing my post and pressing the "submit" button, I had the message telling me that I was not logged on and asking for my log in details. After doing that I get told that I have duplicated my post and that I am being returned to the original thread. Can anyone assist, please?