Re: Weird.
Latest version of Andriod here - but had just been notified that GoogleMail app had updated so, having checked it was legit, I logged back in and it was all fine, believing it was just down to the app updating.
It was a little worrying, though - but Google seem to take security seriously and I normally get separate notifications if there are any changes to the account (the message simply said that due to a change, I'd been logged out for security reasons and needed to log back in).