Squeezed previews in Premiere Pro CS5

Hey everyone,

I've recently switched to Premiere Pro CS5 and I find that when I render a preview, my preview window becomes squeezed. Black bars appear above and below the window with the image squeezed (and with a lower resolution) in between. I did not have this in CS3. Maybe it has to do with using h.264 Canon 7D footage, I don't know... Anyone have any ideas?

Greetings and thanks,
-Lawant