This notification is sent to the devices signed on with the same Apple ID when one of the devices logs out and back in with the Apple ID entirely or signs in and out of Messages and FaceTime. You will also get a notification if one of the devices activate / reactivate Wi-Fi calling andText Message Forwarding features.
If you did not initiate any of the above actions, then another device may have been used to sign on with your Apple ID and Password. You can doublecheck to see whether there is an unknown device under your Apple ID by opening Settings ➡️ Apple ID (Your Name on Top): You should see all of your devices listed at the bottom. If you see an unknown device, then someone else has your Apple ID and Password. In this even, you should tap on that unknown device and then Remove it from under your Apple ID.
Once this is done then you should change the password to your Apple ID by opening Settings ➡️ Apple ID (Your Name on Top) ➡️ Password & Security ➡️ Change Password.
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