Originally posted by mcgregni Av mode is specifically for locking the aperture value as you want it, so the shutter speed will be changing and possibly the ISO as well .....
To fix shutter speed you should use Tv or M modes.
Exactly. You have set the camera up to only change the shutter speed by using Av.
I really like Av for everything else but when I use the flash, I switch to M. Admittedly this looks easier on a 2-dial camera, because I can set it up so I still can have the shutter speed on one dial and aperture on another. But flash adds another variable, flash power, and you often don't want to change so many things at once. In M mode you can lock things in one place and that works nicer for flash.