did you pump the brake pedal after you pushed the caliper piston in?
if there sufficient brake fluid in the resevoir?
did you perform a bleed after the pads were replaced. if you did, maybe check that the bleed nipples are done up.
if you didnt, maybe it is a good idea to bleed the system, if its feeling a bit spongy.
check that you didnt kink any brake lines, hoses, check for leaks if you have.

either that or you just start noticing the brake pedal sicne you just replaced the pads. its a mental thing. you start paying more attention to something that just got replaced to try and point out a fault.
check the above and come back to us. =]