Probably a phishing scam.
If you're logging into Gmail to view your email, then (clearly) your Google sign-in can't have changed or you wouldn't be able to read the email that tells you it has.
To check if the email is genuine though, go into its 'extended headers' (a.k.a. 'full headers'). The way to do so varies across different email systems though. This tells you how to do it in Gmail (and some other systems too):
https://support.google.com/mail/answer/29436
Using GMX, as I do, though, all that's necessary is to click on the tiny little 'i' (for 'information') that appears at the top of an email.
Once you've got into the extended headers, look at both the 'Return-path' field and the 'Received' field. They should both contain references to 'google.com'. If, instead, you see 'gmail.com' (or any other email service) in there, the email is definitely NOT from Google.