Project is letterboxed on the stage all of a sudden?

Hi

I'm working on an iOS AIR project in Flash Pro cs6.

I was doing my normal thing, and then all of a sudden, the display is squeezed in with about an inch of black on the left and right sides.

I'm guessing I've pressed a key that's chnaged something globally, but I have no idea what.

Anyone know what can set this off? My publish settings are full screen, 640w x 940h, and I didn't change anything in my code when the change happened.

Cheers