Soooo....that's what happens when you install new pads, you've never changed brakes before?
Remove the pads and clamp down on the piston with a C-clamp until it's flush with the caliper, reassemble brakes and they should magically fit again :p


Don't worry, it happens to the best of us...we get so sidetracked thinking about how stuff works in other areas that we forget some of the most obvious stuff out there.