iPhone 4 - keeps asking for Apple ID password
I've had the phone for at least 18 months. In the last week for some reason, the phone continually asks me to enter my Apple ID password. This happens every time I unlock the phone, and often while i'm in the middle of using an application. If I cancel the request, the dialog box comes back a few seconds later. If I enter my apple ID it goes away until next time I want to use the phone.
Help please. Might I have inadvertently activated something that requires me to enter my Apple ID password every time I use the phone? Is there something I can turn off to fix this. I've tried shutting down and restarting the phone.
Thanks for any suggestions.