I got a message on my mobile today:
"There's been a change to your Google account"
along with a prompt to re-enter my password.
Could someone be using my account or is this a known issue?
It looks like this is a known issue, and Google is on top of it.
From the product forums:
We've gotten reports about some users being signed out of their accounts unexpectedly. We're investigating, but not to worry: there is no indication that this is connected to any phishing or account security threats.
Please try to sign-in again at accounts.google.com and if you cannot remember your password, please use this link (g.co/recover) to recover your password. If you use 2-Step Verification, there may have been a delay in receiving your SMS code. Please try again or use backup codes.
Essentially the same message is currently displayed at the top of the Accounts Help: Can't sign in to your Google Account page.
Also, from @google on Twitter:
We know some of you had issues signing in today. Please try again now. Rest easy -- your account's security was not affected.