its internet droping maybe or someone has your steam password either way your session has been disconnected from steam (its a good idea to have console open cus it says about achivments not being earned if its a different server.dll or disconnected it says)

It can be caused by your Internet connection dropping out or someone else logging into your Steam account. Either way your session has been disconnected from Steam.

It is a good idea to have the console open because it tells you when achievements are not being earned if it's a different server.dll or you are disconnected.