registered, I doubt that there's anything sinister going on.
Are you sure that you don't have any Amazon devices at home such as a Fire tablet or even their TV stick?
I have changed my ISP password as a precaution.
That won't stop unauthorised devices from connecting to your router. If you're still worried then login to your home router and change the wireless password. Note that afterwards you'll need to reconnect with that new password on all your wireless devices.