Changing Itunes store problems

I live in Japan but have been using the U.S. Itunes store since I got my Iphone 4.  For some reason, a few days ago the Itunes store on my Iphone was switched to the Japan Store.  When I try to switch back to U.S. store, the Apple ID button on all my apps (Itunes, Podcasts, App Store, etc.) is grayed out and won't function.

As far as I know my phone hasn't been hacked or "locked out" because I can log into Itunes Store going through Itunes on my laptop.

Any ideas on what's going on? 

Simply put, I need a way to change from Itunes Store Japan to Itunes Store U.S.

thanks