Its a bug in the steam system where if you're logged on for a certain amount of time or someone else logs into your account. It may ask you to re-longin. Actually it should ask you to. Just restart steam. If it continues persue it via the steam forums and send them a tech request.
yea don't worry about it, i myself sometimes get that message, just close your steam and turn it back on, if it doesn't let you play wait for 5-10 mins and try again.