Google released a statement earlier today, confirming that it was not a security breach. I found an article here
Here's their statement:
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.