password POP UP

All of a sudden I keep getting this constant and very annoying PopUp:

Sign-In required

Enter password for xyz@gmail.com (The xyz would be my Apple ID)

When I enter my Apple ID password it says something like ‘password failed please try again‘ I am NOT entering the wrong password so what gives? How can I make this GO AWAY? But more importantly what caused it to start popping up in the first place?

Please Help!

I have an iPhone 7 Plus iOS 11.3.1