Why is my phone logged into a stranger's Google account?
Could it be that you're logged in the same account as before, but the stranger has managed to log in to your account, and has started to make it his/hers? Try to log in to your account using a browser on a computer, and make sure that your password still works and that the account is as you made it to be. Another thought: are you sure it's your phone? Could it have been swapped with the stranger while you were out somewhere?