This Occurs every time the IP address you log in with Changes. This is designed to prevent anyone without access to your User ID, Password and Email Address and Password from stealing your account.

I see this message often because I Play on four different computers.

If you are seeing this constantly from a single computer it is likely that your Service Provider does not give you a static IP address and has been changing it on you on a regular basis. This is a fairly normal practice, but I believe you can manually override it in your router settings.