May I ask what browser you're using? Some do seem to log members out automatically, but it hasn't happened in a while that I have heard of. Some clear all cookies automatically within a certain amount of time, and with them they sometimes log you out of sites you're logged into. I would suggest checking your settings and see if you have enabled the option to automatically clear them. If you do, you might be able to fiddle a bit with it.
Actually, the first thing I'd suggest you try is to log out, clear your cache, and then log back in. See if that helps anything.
Hope this helps.
Regards,
Der Strom